Cancer Articles

Night Shift Might Boost Women's Breast Cancer Risk

Night Shift Might Boost Women’s Breast Cancer Risk

By Steven Reinberg
HealthDay Reporter
MONDAY, May 28 (HealthDay News) — Women who work the night shift more than twice a week might be increasing their risk for breast cancer, Danish researchers find.
Moreover, the risk appears to be cumulative and highest among women who describe themselves as “morning” people rather than “evening” people, the researchers say.
“About 10 to 20 percent of women in modern societies have night shift work,” said lead researcher Johnni Hansen. “It might therefore be one of the largest occupational problems related to cancer.”
Right now, the reasons for these findings are uncertain.
“Night shift work involves exposure to light at night, which decreases the production of the night hormone melatonin that seems to protect against certain cancers,” said Hansen, of the Institute of Cancer Epidemiology at the Danish Cancer Society, in Copenhagen.
In addition, light at night might introduce circadian disruption, where the master clock in the brain becomes desynchronized from local cellular clocks in different body organs, affecting the breast, he said.
“Repeated phase shifting may lead to defects in the regulation of the circadian cell cycle, thus favoring uncontrolled growth,” Hansen said.
Also, sleep deprivation after night shift work leads to the suppression of the immune system, which might increase the growth of cancer cells, he added.
This is not the first time this association has been recognized. In 2007 the International Agency for Research on Cancer, part of the World Health Organization, said that working the night shift is “probably carcinogenic to humans,” according to background information in the study.
The new study was published in the May 28 online edition of Occupational and Environmental Medicine.
To determine the effect of night shift work on the risk for breast cancer, Hansen’s team collected data on more than 18,500 women who worked for the Danish Army between 1964 and 1999.
The researchers identified 210 women who had breast cancer and compared them to almost 900 similar women who did not have breast cancer.
All of the women were asked about their working patterns, lifestyles and other factors such as their use of contraceptives and hormone replacement therapy, and their sunbathing habits.
In addition, women were asked to classify themselves as “morning” or “evening” people.
In all, 141 women with breast cancer responded to the study questionnaires. In addition, 551 women who did not have breast cancer responded.
Among these women, the risk for breast cancer was increased 40 percent if they worked at night, the researchers found.
But for women who worked nights at least three times a week, and for at least six years, the risk was doubled, the findings showed.
Women who worked the night shift but who described themselves as morning people were at even higher risk of breast cancer. They were almost four times more likely to develop breast cancer as those who didn’t work nights, according to the researchers.
In comparison, women who considered themselves evening people were twice as likely to develop breast cancer, they added.
Morning-preferring women who did not work at night had a lower overall risk of breast cancer than evening types, Hansen’s team found.
“Since night shift work is unavoidable in modern societies, this type of work should be limited in duration and limited to less than three night shifts per week,” Hansen said. “In particular, morning types should limit their night work,” he added.
While the study found an association between night shift work and breast cancer, it did not prove a cause-and-effect relationship.
Men who work at night may also be at risk for prostate cancer, Hansen noted. This evidence comes from three small studies, he said.
Dr. Stephanie Bernik, chief of surgical oncology at Lenox Hill Hospital in New York City, said that “it’s very hard to single out the causes of breast cancer and whether women on the night shift have a higher risk.”
But there appears to be some truth to this, she added.
“Stress increases the risk of breast cancer and affects the body as a whole, and working at night can throw off your circadian rhythm and cause stress,” Bernik said.
“This is another finding that breast cancer is caused by a multitude of different environmental and genetic influences, so this is probably a piece of the puzzle,” she added.
An expert on environmental factors that affect cancer, Richard Stevens, from the department of community medicine at the UConn Health Center in Farmington, Conn., said that “the evidence is growing rapidly about light at night and, specifically, shift work and breast cancer.”
“But, this is the first study about the morning/evening preference,” he noted.
“If it’s true that light at night increases the risk of disease, then there are very practical implications,” Stevens said.
If lighting is really an issue in night work, it is known which wavelengths suppress melatonin the most, and lighting could be adjusted to eliminate those wavelengths, he explained.
There are other things people can do to avoid the effects of light at night on health, he added.
“For example, for health in general, if you wake up during the night, stay in the dark; don’t turn on the light. If you turn on the lights it will start suppressing melatonin immediately,” Stevens said.
“There is a lot more involved than melatonin, but it’s a good marker if your circadian rhythm is being changed,” he said.
Studies are ongoing on the risk of night work and prostate cancer, Stevens noted.

Blue Light Has A Dark Side

Blue Light Has A Dark Side

Harvard Health Letter

Harvard Medical School

Light at night is bad for your health, and exposure to blue light emitted by electronics and energy-efficient lightbulbs may be especially so.

Until the advent of artificial lighting, the sun was the major source of lighting, and people spent their evenings in (relative) darkness. Now, in much of the world, evenings are illuminated, and we take our easy access to all those lumens pretty much for granted.

But we may be paying a price for basking in all that light. At night, light throws the body’s biological clock—the circadian rhythm—out of whack. Sleep suffers. Worse, research shows that it may contribute to the causation of cancer, diabetes, heart disease, and obesity.

But not all colors of light have the same effect. Blue wavelengths—which are beneficial during daylight hours because they boost attention, reaction times, and mood—seem to be the most disruptive at night. And the proliferation of electronics with screens, as well as energy-efficient lighting, is increasing our exposure to blue wavelengths, especially after sundown.

Daily rhythms influenced by light

Everyone has slightly different circadian rhythms, but the average length is 24 and one-quarter hours. The circadian rhythm of people who stay up late is slightly longer, while the rhythms of earlier birds fall short of 24 hours. Dr. Charles Czeisler of Harvard Medical School showed, in 1981, that daylight keeps a person’s internal clock aligned with the environment.

The health risks

Study after study has linked working the night shift and exposure to light at night to several types of cancer (breast, prostate), diabetes, heart disease, and obesity. It’s not exactly clear why nighttime light exposure seems to be so bad for us. But we do know that exposure to light suppresses the secretion of melatonin, a hormone that influences circadian rhythms, and there’s some experimental evidence (it’s very preliminary) that lower melatonin levels might explain the association with cancer.

A Harvard study shed a little bit of light on the possible connection to diabetes and possibly obesity. The researchers put 10 people on a schedule that gradually shifted the timing of their circadian rhythms. Their blood sugar levels increased, throwing them into a prediabetic state, and levels of leptin, a hormone that leaves people feeling full after a meal, went down.

Even dim light can interfere with a person’s circadian rhythm and melatonin secretion. A mere eight lux—a level of brightness exceeded by most table lamps and about twice that of a night light—has an effect, notes Stephen Lockley, a Harvard sleep researcher. Light at night is part of the reason so many people don’t get enough sleep, says Lockley, and researchers have linked short sleep to increased risk for depression, as well as diabetes and cardiovascular problems.

The power of the blues

While light of any kind can suppress the secretion of melatonin, blue light does so more powerfully. Harvard researchers and their colleagues conducted an experiment comparing the effects of 6.5 hours of exposure to blue light to exposure to green light of comparable brightness. The blue light suppressed melatonin for about twice as long as the green light and shifted circadian rhythms by twice as much (3 hours vs. 1.5 hours).

In another study of blue light, researchers at the University of Toronto compared the melatonin levels of people exposed to bright indoor light who were wearing blue-light–blocking goggles to people exposed to regular dim light without wearing goggles. The fact that the levels of the hormone were about the same in the two groups strengthens the hypothesis that blue light is a potent suppressor of melatonin. It also suggests that shift workers and night owls could perhaps protect themselves if they wore eyewear that blocks blue light. Inexpensive sunglasses with orange-tinted lenses block blue light, but they also block other colors, so they’re not suitable for use indoors at night. Glasses that block out only blue light can cost up to $80.

Less-blue light

If blue light does have adverse health effects, then environmental concerns, and the quest for energy-efficient lighting, could be at odds with personal health. Those curlicue compact fluorescent lightbulbs and LED lights are much more energy-efficient than the old-fashioned incandescent lightbulbs we grew up with. But they also tend to produce more blue light.

The physics of fluorescent lights can’t be changed, but coatings inside the bulbs can be so they produce a warmer, less blue light. LED lights are more efficient than fluorescent lights, but they also produce a fair amount of light in the blue spectrum. Richard Hansler, a light researcher at John Carroll University in Cleveland, notes that ordinary incandescent lights also produce some blue light, although less than most fluorescent lightbulbs.

What you can do

  • Use dim red lights for night lights. Red light has the least power to shift circadian rhythm and suppress melatonin.
  • Avoid looking at bright screens beginning two to three hours before bed.
  • If you work a night shift or use a lot of electronic devices at night, consider wearing blue-blocking glasses.
  • Expose yourself to lots of bright light during the day, which will boost your ability to sleep at night, as well as your mood and alertness during daylight.

Light at Night and Melatonin

 Light at Night and Melatonin

Excellent excerpt on LAN and melatonin from State of the Evidence: The Connection Between Breast Cancer and the Environment (sixth edition, 2010), by Janet Gray, Ph.D. 

Light at Night and Breast Cancer Risk Worldwide.

Light at Night and Breast Cancer Risk Worldwide.
12/1/2010Authors:Spivey, AngelaSource:Environmental Health Perspectives; Dec2010, Vol. 118 Issue 12, pA525-A525, 1pDocument Type:ArticleSubject Terms:*BREAST — Cancer
*LIGHT — Physiological effect
RISK factorsNAICS/Industry Codes:335129 Other Lighting Equipment ManufacturingPeople:STEVENS, RichardAbstract:The article offers information on the relation of light at night (LAN) on increased risk of breast cancer. It mentions studies including those of epidemiologist Richard Stevens and says that LAN’s relation on increased breast cancer risk may be driven by its ability disrupt human circadian system. It says that other studies suggest the connection of melatonin and lower clock gene expression to reduce risk of breast cancer. Meanwhile, the amount of LAN cancer risk contribution remains unknown. INSET: Embrace the dark… for better health.

Connection Between Light at Night (LAN) and Cancer Revealed in Additional Study

Connection between light at night (LAN) and cancer revealed in additional study
9/2/2010Bioscience Technology  9/2/2010Rachel FeldmanA new study from the Center for Interdisciplinary Chronobiological Research at the University of Haifa has found an additional link between Light At Night (LAN) and cancer. This research joins a series of earlier studies carried out at the University of Haifa that also established the correlation. “High power light bulbs contribute more to ‘environmental light pollution’, which the study has shown is a carcinogenic pollution,” notes Prof. Abraham Haim, who headed the study.Earlier studies in which Prof. Haim has participated at the University of Haifa, have shown that people living in areas that have more night-time illumination are more susceptible to prostate cancer in men and breast cancer in women. The researchers’ hypothesis was that LAN harms production of melatonin, a hormone that is released from the pineal gland during the dark part of the 24h cycle and which is linked to the body’s cyclical night-day activity and seasonality. When this hormone is suppressed, the occurrence of cancer rises.The current study, in which Dr. Fuad Fares and Adina Yokler, Orna Harel and Hagit Schwimmer also participated, set out to establish or refute this hypothesis. In order to do so, four groups of lab mice injected with cancerous cells were examined: one group was exposed to “long days” of 16 hours of light and 8 hours of darkness, simulating exposure to artificial light beyond the natural number of light hours in a day; a second group was exposed to the same “long days” but were treated with melatonin; a third group was exposed to “short days” of 8 light hours and 16 dark hours; and a fourth group was exposed to the same “short days” but during the dark hours was exposed to a half-hour interval of light.The results show once again the clear link between LAN and cancer: the cancerous growths in mice exposed to “short days” were smallest (0.85 cubic cm. average), while those mice exposed to the interval of LAN during dark hours had larger growths (1.84 cubic cm. average) and those exposed to “long days” even larger growths (5.92 cubic cm. average).The study also discovered that suppression of melatonin definitely influences development of the tumor. The size of tumor in mice exposed to “long days” but treated with melatonin was only 0.62 cubic cm. on average, which is not much different from the size of the growth in mice exposed to “short days”. The study also found that the death rate in mice treated with melatonin was significantly lower than in those not treated.The researchers say that their study results show that suppression of melatonin due to exposure to LAN is linked to the worrying rise in the number of cancer patients over the past few years. However, it is not yet clear what mechanism causes this.”Exposure to LAN– disrupts our biological clock and affects the cyclical rhythm that has developed over hundreds of millions of evolutionary years that were devoid of LAN. Light pollution as an environmental problem is gaining awareness around the world, and the World Health Organization’s International Agency for Research on Cancer (IARC) has already classified working the night shift as a higher grade of cancer risk,” the researchers noted.

Shedding a Little Light on Pollution

Shedding a Little Light on Pollution

Shedding a Little Light on Pollution

Edge on Health Blog

August 11th, 2010   By Dr. Ann Louise

What you don’t know about lighting can hurt you.

Global warming may still be up for debate. But there’s no question that the world’s getting brighter—due to the proliferation of both indoor and outdoor lights.

“Darkness is as essential to our biological welfare, to our internal clockwork, as light itself,” writes Verlyn Klinkenborg in a recent National Geographic article. Sleep scientists find that bathing our world in artificial light not only disrupts our body clocks but also disturbs hormone production.

One recent study links higher rates of breast cancer in women with night-time brightness in their neighborhoods. And numerous studies find an increased risk for cancers—prostate as well as breast—among night-shift workers. Experimental evidence suggests that the antioxidant hormone melatonin, critical for sound sleep and healthy immunity, plays a major role in increased cancer risk.

One interesting investigation published in Medical Hypotheses suggests that artificial—versus natural—light not only inhibits melatonin production but also increases the circulating estrogen levels, increasing the risk for melanoma. French researchers note the therapeutic effect of melatonin in metastatic diseases, and seasonal patterns in melanoma are consistent with seasonal melatonin production in the body.

A 2009 New York Academy of Sciences conference paper implicated artificial lighting in cancer, depression, diabetes, and obesity. And certain types of fluorescent lighting can be particularly damaging.

In addition to health problems—ranging from headaches and eye problems to electromagnetic radiation—linked to fluorescent lighting, a new study at the University of Pennsylvania finds that long-term exposure can cause cumulative skin damage in people with lupus and other photosensitive skin disorders. Those new energy-efficient compact bulbs add even another risk—the potential for mercury pollution.

Dr. Ann Louise’s Take:

Light pollution may be the easiest environmental hazard to remedy. But it’s all too easily overlooked in today’s 24/7 world where people text into the wee hours and when global climate change seems a far more pressing issue.

I’ve been concerned about light pollution for some time now. The typical American gets only about 7 to 8 hours of darkness every night—and probably only secretes melatonin 6 to 7 of those hours. Without today’s light pollution, though, we’d be producing this critical hormone for 9 to 10 hours a night!

Most indoor lighting ranges from 100 to 1,000 lux (a measurement of how light illuminates). Studies show that as little as 100 lux at night can inhibit melatonin production—and 500 lux (easily achieved in our brightly lighted homes and urban centers) can impair melatonin production up to 98%.

The Right Light
Then there are specific risk factors depending on the type of artificial light you’re exposed to. For example, energy efficient fluorescents—whether those long tubes or the compact fluorescent lights (CFL)—create elevated magnetic and electric fields whenever they’re on.

Research at University of Oklahoma College of Medicine also finds that compact fluorescent bulbs—unless they come with shields—leak ultraviolet B radiation, which is key in promoting skin cancer. Is this the lighting you want next to your—or your child’s—head?

Break one of these corkscrew-shaped CFLs—and you’ve got a Hazmat crisis right in your own home! That’s because the mercury in just one bulb can contaminate a whole room. Recycling them isn’t easy, either. In a landfill near your home, they leach mercury in groundwater and soil.

In addition to fluorescents, halogen lights (which generate a lot of heat and require ventilation) and even the electrical wiring in your home can produce significant amounts of electromagnetic fields (EMFs), increasingly found hazardous to human health. A low voltage DC halogen light that’s set into the ceiling is safe if you’re beneath it, but it can radiate high magnetic fields about three feet into the floor above, so don’t place them underneath rooms where children sleep or play. In addition, the AC magnetic fields produced by lights and ceiling fans are greater in the rooms above them because you are closer to the source.

For safety’s sake, the classic incandescent bulb and LEDs are best. There’s a new generation of these lights coming to market that meets the tough 2012 efficiency standards, and Philips makes an energy-efficient incandescent bulb (really a halogen lamp in incandescent clothing) that’s already available at Home Depot and on Amazon.

Up Your Melatonin
This hormone enhances the power of the immune system’s lymphocytes to fight cancer cells that slip by the body’s other defenses, so it’s hardly surprising that low melatonin levels have been implicated in breast cancer, melanoma, and malignancies of the ovary and prostate. Low levels of melatonin are also linked in Alzheimer’s and Parkinson’s diseases.

Even low levels of EMFs can depress melatonin production, so I advise trying some of the detection devices I’ve found useful in my own home. Go to for handy, reasonably priced ways to discover—and if necessary, re-mediate—dangerous radiation in your home or office.

Room-darkening drapes and shades won’t reduce your exposure to electrical and magnetic field radiation. But keeping your room darker longer can help your body produce more melatonin, which is also diminished by exposure to EMFs.

In studies of older people (who tend to have lower than normal levels of melatonin and consequently have trouble sleeping), those who were exposed to about two hours of sunlight a day had higher levels of melatonin—and slept better—than those who didn’t spend as much time in the sun. Even being outside on a cloudy day or sitting in the shade appears useful.

Blue light helps keep you alert, but it also suppresses melatonin production in the evening. Researchers with the Lighting Innovations Institute at John Carroll University in Ohio have developed reasonably priced blue-blocking glasses, light bulbs, and night lights that their studies found helped promote better sleep and reduced symptoms of ADHD in people who wore them a few hours before bedtime (see resources). has special glasses and nightlights that block out blue light that inhibits melatonin production. I own a pair of these glasses and wear them an hour before bed, which helps me sleep through the night without waking.

Because melatonin regulates the sleep-wake cycle and supports other antioxidants as well as vital immune system components called natural killer cells, also consider melatonin supplements (1 to 3 mg in a time-release formula). Tart red cherries are an excellent food source of this antioxidant hormone.

Added Support
In one placebo-controlled study, Jeffrey Cram, PhD, tested the effects of a specific flower essences—Yarrow Special Formula and Five-Flower Formula—on people exposed to fluorescent lights. While the placebo group exhibited increased agitation from these artificial light frequencies —those subjects who took one or the other of the flower essences did not!

Denmark Pays Compensation for Breast Cancer After Night-Shift Work

Denmark Pays Compensation for Breast Cancer After Night-Shift Work
3/23/2009From Medscape Medical NewsDenmark Pays
Compensation for Breast Cancer After Night-Shift Work
Zosia Chustecka







March 23, 2009 — Denmark has
become the first country to pay government compensation to women who developed
breast cancer after long spells of working at night.


The ruling, reported
last week by many media outlets as news, was actually made last year.


The Danish National
Board of Industrial Injuries explains on its Web site
that in 2008, breast cancer after night-shift work was recognized as an
industrial injury in 38 of 75 cases that were submitted to the Occupational
Disease Committee. Compensation was granted in all but 1 of the 38 cases, and
was paid by the employer’s industrial-injuries insurance.


The cases that won
compensation involved women who typically worked at least 1 night a week for at
least 20 to 30 years, and where there were “no other significant factors
that might explain the development of breast cancer,” the Board said.


Some of the cases
were dismissed because of a strong family history of breast cancer, according
to news reports. The cases included women who had worked as air hostesses and
as nurses, they noted.


One of the women who
received compensation was Ulla Mahnkopf, who had worked for 30 years as a
flight attendant on the Scandinavian airline SAS. In an interview with BBC Scotland, she said that after years of
sleep disruption and jet lag, when she stopped work she felt as if she was
“coming out of a shell.” She was diagnosed with bilateral breast


Based on
Epidemiologic Evidence


The Danish Board
says these claims were recognized after the Occupational Diseases Committee
discussed — in November 2007 — a review of the epidemiologic evidence between
night-shift work and the risk for breast and other cancers.


“After these
discussions, the Committee decided that breast cancer, after many years of
night-shift work, qualified for recognition as an industrial injury,” it
explains on its Web site.


The next step is for
the Board to review the work of the International Agency for Research on Cancer
(IARC) in this field and to decide if breast cancer after night-shift work
should be included on the list of occupational diseases.


The IARC, an agency
of the World Health Organization, has recently classified shift work as a
possible carcinogen.


In a statement
released in December 2007, the IARC said that its expert working group had
concluded that shift work that involves circadian disruption is “probably
carcinogenic to humans,” and it was ranked in group 2A, along with
ultraviolet light radiation. This is below the group 1 category, which is
“carcinogenic to humans,” and includes asbestos, but above
group 2A, which is “possibly carcinogenic to humans,” and
includes lead, the pesticide DDT (dichloro-diphenyl-trichloroethane), and engine exhaust.


“The level of
evidence is really no different than it might be for an industrial
chemical,” Vincent Cogliano, PhD, of the IARC commented in a BBC news


The agency reached
their conclusions after looking at a wide number of studies of both humans and
animals, he explained. One of the studies (J Natl Cancer Inst. 2001;93:1563-1568) showed a 36% greater risk for
breast cancer in women who had worked night shifts for more than 30 years than
in women who had never worked nights.


Dr. Cogliano said
there was evidence to support the hypothesis that alterations in sleep patterns
could suppress the production of melatonin in the body. “Melatonin has
some beneficial effects in preventing some of the steps leading to
cancer,” he explained.


However, 1 breast
cancer expert has questioned the link to night-shift work. John Boyages, MD,
director of the New South Wales Breast Cancer Institute, in Sydney, Australia,
said he was not convinced. “I am a little bit puzzled, to be honest, about
the compensation case, and I really think [the Danes] have gone out on a
limb,” he said in radio program from the Australian Broadcasting Corporation


“My personal
view is that it is a complex Rubik’s Cube and we might be looking at this as a
night-shift issue, but when you turn the cube the other way, it is probably
obesity, it might be [hormone replacement therapy], it might be a lack of
vitamin D, which would be my strong guess,” he told ABC.


Light Pollution Raises Risks of Breast Cancer

Light Pollution Raises Risks of Breast Cancer

Light pollution also wastes energy, disrupts bird and animal breeding cycles

By Larry West,

Dear EarthTalk: What is “light pollution?” Is it really a factor in breast cancer?
Gudrun Smythe, Madison, Wisconsin

The glow of city lights blotting out stars in the night sky has frustrated many a stargazer, but recent studies have shown that “light pollution”—defined as excess or obtrusive light at night—can actually have serious health effects. Researchers have found that exposure to bright nocturnal light can decrease the human body’s production of melatonin, a hormone secreted at night that regulates our sleep/wake cycles. And decreased melatonin production has in turn been linked to higher rates of breast cancer in women.

“Light at night is now clearly a risk factor for breast cancer,” says David Blask, a researcher at the Cooperstown, New York-based Mary Imogene Bassett Research Institute. “Breast tumors are awake during the day, and melatonin puts them to sleep at night.”

Light Pollution Leads to More Breast Cancer in Industrialized Countries
Epidemiologist Richard Stevens of the U.S. Department of Energy’s Pacific Northwest National Laboratory first discovered the link between breast cancer and light pollution in the late 1980s. Stevens found that breast cancer rates were significantly higher in industrialized countries, where nighttime lighting is prevalent, than in developing regions.

Night Shift Workers Run Higher Risk of Breast Cancer from Light Pollution
Lending credence to Stevens’ research are the findings of another researcher, William Hrushesky of the South Carolina-based Dorn Veterans Affairs Medical Center, who discovered that female night shift workers have a 50 percent greater risk of developing breast cancer than other working women. He also found that blind women have high melatonin concentrations and unusually low rates of breast cancer.

How to Reduce Risks of Breast Cancer from Light Pollution
To reduce breast cancer risks from light pollution, Prevention magazine recommends nine hours of sleep nightly in a dark room devoid of both interior (computer screens) and exterior (street lamps) light sources. A study of 12,000 Finnish women found that those who slept nine hours nightly had less than one-third the risk of developing a breast tumor than those who slept only seven or eight hours. Even bright light from a trip to the bathroom can have an affect, so dim nightlights are recommended for night lighting.

How Light Pollution Affects Birds and Animals
Light pollution causes other problems besides increased cancer risks. According to the Sierra Club1, birds and animals can be confused by artificial lighting, leading them away from familiar foraging areas and disrupting their breeding cycles. And the photosynthetic cycles of deciduous trees (those that shed their leaves in the fall) have been shown to be disrupted due to the preponderance of artificial nighttime lights.

Light Pollution and Wasted Energy
Another environmental impact of excessive use of artificial light is, of course, energy waste. The International Dark-Sky Association2 computes that unnecessary nighttime lighting wastes upwards of $1.5 billion in electricity costs around the world each year while accounting for the release of more than 12 million tons of carbon dioxide, the leading greenhouse gas, into the atmosphere. Individuals can do their part by keeping lights dim or turned off at home at night—and convincing their employers and local government offices to do the same.

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P.O. Box 5098, Westport, CT 06881; submit it at:, or e-mail: [email protected].

EarthTalk is a regular feature of E/The Environmental Magazine. Selected EarthTalk columns are reprinted on About Environmental Issues by permission of the editors of E.

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Night Light Raises Cancer Risk

Night light raises cancer risk

THE JERUSALEM POST – Judy Siegel-Itzkovich

Exposure to light at night is the most powerful factor in breast cancer besides genetic defects, according to a new University of Haifa study.

Although one can’t easily move to a dark neighborhood, stop using computers and watching TV late at night, or refuse to do shift work, it is advisable to close the shutters when you go to bed, wear eye shades if you can’t darken the bedroom, avoid night lights and lower lights in working environments after sunset.

These recommendations come from new Israeli research just published in Chronobiology International.

Women who live in neighborhoods and streets with strong outdoor lighting at night are more likely to contract breast cancer than those who have minimal nocturnal lighting, according to the research performed by Prof. Avraham Haim, a chronobiologist and expert in evolutionary and environmental biology at the University of Haifa; Itai Kloog, a doctoral student in the natural resources and environmental management department; and Prof. Boris Portnov.

There is also some unpublished evidence that men in well-lit surroundings are at higher risk for prostate cancer, but there was no link between excess light at night and lung cancer, which is caused almost exclusively by smoking.

Kloog, Haim and Portnov overlaid satellite images produced by NASA with geographical data on breast cancer from Israel’s National Cancer Registry. They also processed questionnaires filled out by 100 women with breast cancer and 100 healthy women about a wide variety of socioeconomic, environmental, genetic and other factors and their exposure to light at night. They found that the breast cancer rate in neighborhoods with average night lighting was 37 percent higher than in those with the darkest streets, while the rate was an additional 27% higher in areas with the highest amount of light.

Prof. Tamar Peretz, chairman of the Sharett Institute of Oncology at Hadassah-University Medical Center in Jerusalem’s Ein Kerem neighborhood and one of the country’s leading experts on breast cancer, told The Jerusalem Post that the study was “very interesting and may explain some of the causes of breast tumors. There are many causes, and light at night may be one, but we don’t know how important it is. I would recommend reducing lighting at night. It certainly can’t hurt.”

The findings bolster theories, promoted by studies of night-shift workers such as nurses, that exposure to light when one should be sleeping increases the risk of cancers. Haim told the Post that scientists have suggested the hormone melatonin – produced by the pineal gland in the brain during sleep in darkness – was involved. Melatonin, he said on Wednesday, is a powerful antioxidant that promotes the suppression and minimizes the expression of cancer genes such as BRCA1, BRCA2 and P53.

“Thomas Edison’s invention of the electric lightbulb was great and changed the world,” said Haim, “but what does it do to health? In recent years, there has been an investigation of seasonality of animals. When rodents in the field were exposed to strong light at night, they died after it disrupted their seasonal acclimatization. Separate studies have shown that blind women have significantly lower breast cancer rates than sighted women. Clinical research on night-shift workers has also shown a higher risk of cancer.”

A part of the World Health Organization announced two months ago that it had recognized night-shift work as a “probable carcinogen” and thus as risky as exposure to certain toxic substances. A study a few years ago found that young children who had a light on all night in their rooms were at higher risk for developing nearsightedness, and it may also slow their sexual development.

Non-image-forming photo receptors in the eye’s retina affect the pineal gland, which makes more melatonin nocturnally if there is little or no light. Intensive blue-tinged light, such as that produced by fluorescent bulbs, reduces melatonin production more than other bulbs, although short-waved fluorescent lights are recommended to save energy.

More studies on this subject should be conducted, said Haim, who noted that “light is not only a source of pollution, but also a carcinogen, and this should be taken into consideration.” But he added that it was not the only risk, nor the most important one.

Lights at Night Are Linked to Breast Cancer

Lights at Night Are Linked to Breast Cancer

Study Bolsters Theory About Interference With Production of Key Hormone

By Rick Weiss
Washington Post Staff Writer
Wednesday, February 20, 2008; A04

Women who live in neighborhoods with large amounts of nighttime illumination are more likely to get breast cancer than those who live in areas where nocturnal darkness prevails, according to an unusual study that overlaid satellite images of Earth onto cancer registries.

The finding adds credence to the hypothesis that exposure to too much light at night can raise the risk of breast cancer by interfering with the brain’s production of a tumor-suppressing hormone.

“By no means are we saying that light at night is the only or the major risk factor for breast cancer,” said Itai Kloog, of the University of Haifa in Israel, who led the new work. “But we found a clear and strong correlation that should be taken into consideration.”

Scientists have known for years that rats raised in cages where lights are left on for much of the night have higher cancer rates than those allowed to sleep in darkness. And epidemiological studies of nurses, flight attendants and others who work at night have found breast cancer rates 60 percent above normal, even when other factors such as differences in diet are accounted for.

On the basis of such studies, an arm of the World Health Organization announced in December its decision to classify shift work as a “probable carcinogen.” That put the night shift in the same health-risk category as exposure to such toxic chemicals as trichloroethylene, vinyl chloride and polychlorinated biphenyls (PCBs).

The mechanism of such a link, if real, remains mysterious, but many scientists suspect that melatonin is key. Secreted by the pineal gland in the brain, the hormone helps prevent tumor formation. The body produces melatonin primarily at night, and levels drop precipitously in the presence of light, especially light in the blue part of the spectrum produced in quantity by computer screens and fluorescent bulbs.

In keeping with the melatonin hypothesis, mice in cages with night lighting have normal cancer rates if they get shots of the hormone. And blind women, whose eyes cannot detect light and so have robust production of melatonin, have lower-than-average breast cancer rates.

Kloog and his colleagues took a previously untried approach to testing the link. They obtained satellite data from NASA that showed in great detail how much light was emitted spaceward from neighborhoods throughout Israel.

Although the light levels that reached the satellite were about one-tenth their intensity on Earth, the approach provides an accurate measure of which areas are brighter or darker than others and by how much.

The team then overlaid that map with local statistics on cases of breast cancer and, for comparison, lung cancer, which is caused mostly by smoking and so would not be expected to be linked to light.

After using neighborhood data to correct for other factors that can affect cancer rates, including wealth, ethnicity and the average number of children in families living in those localities, the researchers found no link between night lighting and lung cancer, they report in this week’s online issue of the journal Chronobiology International.

But the researchers found the breast cancer rate in localities with average night lighting to be 37 percent higher than in communities with the lowest amount of light; and they noted that the rate was higher by an additional 27 percent in areas with the highest amount of light.

Abraham Haim, a University of Haifa chronobiologist involved in the study, said the findings raise questions about the recent push to switch to energy-efficient fluorescent bulbs, which suppress melatonin production more than conventional incandescent bulbs. “This may be a disaster in another 20 years,” Haim said, “and you won’t be able to reverse what we did by mistake.” He called for more research before policies favoring fluorescent lights are implemented, and for more emphasis on using less light at night.

Jim Burch, a University of South Carolina epidemiologist and biostatistician familiar with the study, called the approach and findings “fascinating.”

“The study has limitations,” including not measuring levels of indoor lighting, “but it supports the overall idea,” Burch said. “I think there is enough evidence to suggest we ought to be thinking about this more carefully.”


The Bright Side of Darkness: A better night's sleep and avoiding a cancer risk

The Bright Side of Darkness: A better night’s sleep and avoiding a cancer risk

By Richard L. Hansler, PhD

Bright Light, Big Cancer” (Science News Online, January 7, 2006) and “The Light-Cancer Con-nection” (Prevention Magazine January 2006) are titles of recent articles that warn of the danger of using artificial light at night. Warnings actually go back to the 1990s when scientists discovered that blind people have only half the risk of cancer as do their sighted counterparts. There is not only the epidemiological evidence – such as the increased incidence of breast cancer in nurses working night shifts – but there is also experimental evidence from many studies conducted with animals that show a correlation between artificial light and cancer. In this study tumors grew rapidly in animals that were raised with short nights (more artificial light) while tumors did not grow (or grew slowly) in animals raised with long nights (less artificial light).

Using light at night may be hazardous to one’s health due to nerves that run from special sensors in the retina of the eye that are connected to the pineal gland. The pineal gland produces melatonin, an important cancer-fighting hormone produced only when the eyes are in darkness. Melatonin is a powerful antioxidant that counteracts the damaging effects of free radicals produced by radiation and chemical pollutants. Melatonin also blunts the cancer-promoting nature of estrogen, and interferes with the metabolism of materials that feed cancer cells.

By shortening the length of time we spend in darkness, the length of time that melatonin is produced and present in the blood is reduced. Studies show that primitive societies that experience longer periods of darkness due to the absence of artificial lighting show a much lower incidence of cancer. Of course, there are other variables when comparing sophisticated nations and primitive cultures, but mounting evidence shows a correlation between the use of artificial light and increased health risks.

Without artificial light every place on earth would, on average, experience 12 hours of darkness each night. Recent studies at Harvard found that blind people (who are always in darkness) produce melatonin for 9-10 hours a night. Sighted people who were kept in darkness produce melatonin for the same period of time. Thanks to artificial lighting, most Americans are in darkness for only 7-8 hours a night, reducing their melatonin production time to only 6-7 hours each night. As mentioned previously, blind people have a 50 percent less risk of developing cancer.

It should be noted that melatonin is not the only hormone produced by the pineal gland. Pineal-gland production is more complex and includes other secretions. It is yet to be determined whether these secretions also play a significant role in fighting cancer. This may be why simply taking melatonin orally does not appear to be as effective as the body’s making its own.

A definitive experiment recently was completed by highly experienced scientists, Dr. George Brainard, a neurology professor and the director of the Light Research Group at Thomas Jefferson University in Philadelphia, Dr. David Blask, a researcher at the Bassett Research Institute in Cooperstown, NY, and their associates. They examined blood drawn from women under three different conditions: during the day when melatonin was not present in the blood, during the darkness of night when melatonin was present and during the night when the women had been exposed to several hours of light. Melatonin was not present in the last group of women. In another phase of the research this blood was then supplied to human breast tumors growing on rats. This resulted in the tumors growing rapidly. Supplying the tumors with blood with melatonin resulted in slower growth or none at all. Dr. Blask commented that melatonin-rich blood, “put the tumors to sleep.”

What can be done with this knowledge? Since it seems unlikely that we will not soon give up using electric lights, it is especially encouraging to know that about five years ago it was discovered that not all spectrums of light suppress melatonin production. Only light in the blue end of the visible spectrum causes suppression, so blocking the blue light alleviates the problem. In practical use, other “colors” of light are okay to use for reading, watching television or working on a computer. This dynamic was first demonstrated at the University of Toronto when subjects working a simulated night shift wore goggles that blocked the blue light. The subjects continued making melatonin even though they were in brightly-lit surroundings.

The Lighting Research Institute at John Carroll University in Cleveland, Ohio, has developed light bulbs of various types with coatings that block blue light. There also are filters for television and computer screens and even eyeglasses that block the melatonin-suppressing blue light. People can wear these glasses a few hours before bedtime. Combining this “virtual darkness” with darkness experienced during sleep, the 9-10 hours of melatonin flow may be obtained that, hopefully, will reduce the risk of cancer.

There is another huge extra benefit to blocking blue light. Not only is melatonin a cancer-fighting hormone, it also is the sleep hormone. Melatonin in the blood makes us drowsy and wanting to drop off to sleep. Wearing glasses that block blue light a few hours before bedtime allows melatonin to be present in quantity at bedtime. Sleep comes very quickly, and people who have used the glasses report sleeping more. With 46 million prescriptions written last year for sleeping pills, the use of blue-blocking glasses may be a better method of treating insomnia, without the harmful side effects that drugs can cause.

Dr. Hansler graduated from the University of Chicago and received a Ph.D. in physics from Ohio State University. Since his retirement from GE in 1996, he has served as the director of the Lighting Innovations Institute at John Carroll University. He also is the executive director of the Light and Health Foundation. For more information, please visit, or Dr. Hansler may be reached by calling (216) 397-1657 or e-mailing [email protected].

The Claim: Artificial Light Can Increase Breast Cancer

The Claim: Artificial Light Can Increase Breast Cancer

The New York Times

April 25, 2006

The Claim: Artificial Light Can Increase Breast Cancer

THE FACTS Scientists have known for some time that night shift workers are prone to sleep disorders and trouble staying awake. But when epidemiologists compared people who worked at night with daytime workers several years ago, they found something unexpected: female nightshift workers had elevated rates of breast cancer.

Socioeconomic factors or the stress of burning the midnight oil may play a role. But scientists say the findings may reflect the hazards of constant exposure to artificial light, which disrupts circadian rhythms and throws hormone levels out of whack.

That explanation appears to have some support. Various studies have found that night workers have chronically low levels of melatonin, a hormone that is switched on by darkness and has been shown to suppress tumor growth. A 2004 study, by researchers at Brigham and Women’s Hospital in Boston and Harvard Medical School, found that women who regularly worked night shifts, compared with those who never did, also had significantly elevated levels of estrogen, which can fuel breast cancer.

The same group of researchers also led one of the largest epidemiological studies linking night shifts to breast cancer. That study, published in The Journal of the National Cancer Institute in 2001, followed more than 78,000 nurses for a decade and found that those who worked the most graveyard shifts were nearly 1.5 times more likely to develop breast cancer.

Dr. Mark Rea, director of the Lighting Research Center at Rensselaer Polytechnic Institute, warned that the evidence is circumstantial and experts note that correlational studies do not prove cause and effect.

THE BOTTOM LINE Circumstantial evidence suggests prolonged exposure to artificial light may raise breast cancer risk in women who work night shifts.

Bright Lights, Big Cancer, Melatonin-Depleted Blood Spurs Tumor Growth

Bright Lights, Big Cancer, Melatonin-depleted blood spurs tumor growth

Bright Lights, Big Cancer

Melatonin-depleted blood spurs tumor growth

Ben Harder

In late 1987, Richard G. Stevens, then at Pacific Northwest Laboratories in Richland, Wash., typed up a short letter and mailed it to Walter Willett at Harvard Medical School in Boston. The two epidemiologists had met just once, and Stevens wasn’t confident that his 209-word note, or the suggestion that it contained about a possible contributor to breast cancer, would inspire any action.

But Willett took the suggestion seriously. He and his colleagues began a study that only they could do. They run the Nurses’ Health Study, a project unrivaled in scope and duration that tracks how women’s health relates to diet, activity, and other factors.

Several years later, members of Willett’s team reported that women who frequently work night shifts seem predisposed to develop breast cancer.

It was just as Stevens had suspected. He had hypothesized that nighttime illumination, by interrupting the body’s mainly nocturnal production of the hormone melatonin, might increase the risk of breast cancer. Animal experiments and surveys of people over the past 2 decades supported that hypothesis without proving it, says Stevens, currently at the University of Connecticut Health Center in Farmington.

“Now, a watershed study has provided the first strong experimental support,” Stevens says.

A woman’s blood provides better sustenance for breast cancer just after she’s been exposed to bright light than when she’s been in steady darkness, researchers led by David E. Blask of the Bassett Research Institute in Cooperstown, N.Y., report.

“Light at night is now clearly a risk factor for breast cancer,” Blask says. “Breast tumors are awake during the day, and melatonin puts them to sleep at night.” Add artificial light to the night environment, and “cancer cells become insomniacs,” he says.

“Sleep per se is not important for melatonin,” says Russel J. Reiter, a neuroendocrinologist at the University of Texas Health Science Center in San Antonio. “But darkness is.”

The new study has far-reaching implications, says Reiter. First, it could spawn trials that test whether malignancies can be slowed down by altering a person’s light environment or by using melatonin supplements. Second, he says, similar studies could show whether exposure to nocturnal light poses a prostate cancer risk to men, as some researchers suspect, or promotes other cancers previously linked to light at night (SN: 8/28/04, p. 141: Available to subscribers at

When cancer awakens

Melatonin forms in the pineal gland, located in the brain, and circulates in the bloodstream. Blood concentrations of the hormone rise after dark from low daytime values and usually peak in the middle of the night.

Because the pineal gland responds to signals transmitted by the optic nerves, bombarding a person’s eyes with bright light during the night can erase the usual nocturnal surge and lower the overall melatonin production for the day. That observation concerned researchers, in part because melatonin has slowed breast cancer growth in lab experiments.

Then, there’s the disturbing circumstantial evidence.

“Breast cancer is epidemic in the world. It’s increasing everywhere,” says Stevens. It’s most prevalent in industrialized countries, where electric lights are widely used, he says. “It’s increasing very rapidly in places that are industrializing,” he adds.

Furthermore, compared with other workingwomen, female night-shift workers have about a 50 percent greater risk of developing breast cancer, says William Hrushesky of Dorn Veterans Affairs Medical Center in Columbia, S.C.

Blind women, by contrast, have unusually low rates of breast cancer and high average melatonin concentrations, he says.

“Almost nobody who does shift work adapts to it,” Stevens says. On their days off, most shift workers concentrate their activities during daylight, which upsets their circadian rhythms as much as commuting across several time zones would, he says.

That presumably explains why the original Harvard study of nurses, which was led by Willett’s colleague Eva S. Schernhammer, found that shift workers had an elevated risk of breast cancer (SN: 11/17/01, p. 317: Available to subscribers at

More recently, Schernhammer and her Harvard colleague Susan E. Hankinson found that women who happen to have above-average melatonin concentrations are relatively unlikely to develop breast cancer.

The Harvard researchers estimated nurses’ peak nightly melatonin concentrations by measuring the hormone in the first urine void of a day. “Those with higher levels seem to have lower breast cancer risk,” Schernhammer says. She and Hankinson reported the data in the July 20, 2005 Journal of the National Cancer Institute.

An earlier study didn’t find the same statistical relationship, but it had involved melatonin measurements in urine samples taken later in the day. Such samples are less likely to correlate with nocturnal hormone concentrations, says Schernhammer.

She notes that light is not the only relevant factor. Age and obesity both reduce a person’s melatonin production, and heavy smoking may do the same, she says. She and other researchers will report the first data that support the smoking-melatonin relationship in an upcoming Journal of Pineal Research.

Breast cancer is less common in women who sleep more than 9 hours per night than in women who sleep less, Stevens and six colleagues in Finland report in the Oct. 15, 2005 Cancer Research. They compared cancer incidence in 12,222 Finnish women whose average nightly sleep duration had been recorded in 1975 and 1981. By 1996, 242 of the women had developed breast tumors.

Women who consistently slept 9 or more hours per night had less than one-third the risk of developing a breast tumor than women who slept 7 or 8 hours per night.

Now, Blask and his collaborators at several institutions have pushed beyond studies finding correlations among cancer, light, and melatonin. At Thomas Jefferson University in Philadelphia, researchers led by George C. Brainard asked each of a dozen healthy female medical students to give three blood samples, one during the day and two at night.

The first nighttime blood draw occurred at 2 a.m., after each woman had been in complete darkness for 2 hours. Then the volunteers stared at a brightly lit, white wall for 90 minutes, and the second nighttime draw took place at 3:30 a.m.

As expected, blood from the 2 a.m. samples contained the highest concentrations of melatonin, and daytime blood contained the lowest. Brainard then sent the samples to Blask for an unusual test of their effect on human-cancer cells.

In Cooperstown, Blask and his colleagues had implanted human breast tumor into rats in such a way that a single artery fed a tumor and a single vein received all blood leaving the cancerous tissue. The team then put plastic tubes into the two vessels, creating external conduits to and from the tumor. The researchers also shut the cancerous tissue off from the rest of the rat’s circulatory system.

Next, they pumped each blood sample from the Philadelphia medical students into a separate rat’s arterial tube and collected the liquid as it came out of the cancerous human tissue. By comparing what blood components went in and what came out, the researchers assessed the tumors’ responses to the concentrations of melatonin in the samples. For example, they measured the tumors’ uptake of H3-thymidine, an ingredient of DNA that reflects cell division and replication in a tumor.

The results indicated that the tumor cells divided most rapidly when supplied by blood taken from women either in daylight or at night after exposure to the bright artificial light. Those blood samples had low melatonin concentrations. Spiking the samples with synthetic melatonin removed their capacity to promote cancer.

Moreover, melatonin-rich blood from women who had been in darkness spurred cell division only when the researchers added a chemical that blocks melatonin’s biological activity.

In further experiments, Blask’s team determined that melatonin blocks cancer cells’ metabolism of linoleic acid, a polyunsaturated fat that’s abundant in food. The same team had previously shown that 13-hydroxyoctadecadienoic acid, the product of linoleic acid metabolism, spurs cancer cells to divide.

The team reports its results in the Dec. 1, 2005 Cancer Research.

The unusual test in the rats shows “close to conclusively” that light-induced suppression of melatonin promotes breast-tumor growth, says Schernhammer.

Stevens adds that Blask’s new technique of testing people’s blood on human tumors in animals is a powerful tool for evaluating the effect of all sorts of actions. Eating a particular food or inhaling a pollutant, for example, could alter the blood concentrations of substances that promote or fight cancer.

Managing melatonin

In the United States, synthetic melatonin is sold over the counter as a dietary supplement. Blask and other researchers want to see tests to assess whether the hormone in this form can ward off breast cancer in women. But they warn that it would be premature for people to take the hormone for that purpose.

“I personally would be pretty cautious about taking over-the-counter melatonin supplements,” says Scott Davis, an epidemiologist at the University of Washington in Seattle. “Melatonin supplements are not regulated” the way drugs are, he notes. “There may be all kinds of impurities and contaminants.”

Although synthetic melatonin hasn’t been shown to be dangerous, it could have adverse effects on the production of reproductive hormones, cautions Schernhammer.

Hrushesky is currently testing the possible benefits of melatonin supplements in men who have undergone surgery for prostate cancer. For now, though, he encourages people to opt for commonsense measures to ensure they get nightly melatonin spikes. Those precautions include going to sleep in the dark at a consistent time each night, exercising regularly, and avoiding evening use of melatonin-suppressing substances, including alcohol and medications such as beta-blockers.

People’s behavior after bedtime also counts. “They should avoid even brief intervals of [bright] light at night,” says Reiter. “A nightlight is generally safe,” he adds, because dim light has relatively little effect on melatonin.

Schernhammer offers similar advice: “If [getting up] to go to the bathroom, avoid turning on the light, or keep it dim.”

But other scientists say that it’s unclear how much a quick trip to an illuminated bathroom affects melatonin in the blood. “It’s probably inconsequential,” says Mark Rea, director of the Lighting Research Center at Rensselaer Polytechnic Institute in Troy, N.Y.

Reiter offers some other strategies for maintaining melatonin production. Blue or white light suppresses melatonin more effectively than red or yellow does (SN: 4/16/05, p. 253: Available to subscribers at, so lights could be designed to filter out the offending wavelengths, Reiter says. Or people could strategically don tinted, wraparound glasses to achieve the same result, he says.

Night-shift workers face fundamental challenges, Blask says. “Melatonin works, to a large degree, by inhibiting the cancer cells from taking up linoleic acid,” he says. Cravings for fatty foods frequently assail workers in the middle of the night. As a result, many shift workers consume large amounts of linoleic acid just when their melatonin production is suppressed and unable to protect them from the polyunsaturated fat, he says.

In addition to its direct effect on breast cancer, melatonin may indirectly combat tumor growth, says Davis. Melatonin suppression encourages the ovaries to produce estrogen and other female sex hormones, which support the growth of cancerous or potentially cancerous cells in a woman’s breasts.

If future studies demonstrate such indirect hormonal effects, they’ll reveal yet one more way by which nighttime light exposure feeds cancer.


Blask, D.E., G.C. Brainard, et al. 2005. Melatonin-depleted blood from premenopausal women exposed to light at night stimulates growth of human breast cancer xenografts in nude rats. Cancer Research 65(Dec. 1):11174-11184. Abstract available at

Schernhammer, E.S., and S.E. Hankinson. 2005. Urinary melatonin levels and breast cancer risk. Journal of the National Cancer Institute 97(July 20):1084-1087. Abstract available at

Schernhammer, E.S., et al. In press. Urinary 6-sulfatoxymelatonin levels and their correlations with lifestyle factors and steroid hormone levels. Journal of Pineal Research. Abstract available at

Verkasalo, P.K. . . . R.G. Stevens, et al. 2005. Sleep duration and breast cancer: A prospective cohort study. Cancer Research 65(Oct. 15):9595-9600. Abstract available at

Further Readings:

Harder, B. 2005. Blue light keeps night owls going. Science News 167(April 16):253. Available to subscribers at

______. 2004. Bright nights kindle cancer in mice. Science News 166(Aug. 28):141. Available to subscribers at

Knight, J.A., et al. 2005. Light and exercise and melatonin production in women. American Journal of Epidemiology 162(Dec. 1):1114-1122. Abstract available at

Megdal, S.P. . . . and E.S. Schernhammer. 2005. Night work and breast cancer risk: A systematic review and meta-analysis. European Journal of Cancer 41(September):2023-2032. Abstract available at

Rea, M.S., et al. 2005. A model of phototransduction by the human circadian system. Brain Research Reviews 50(Dec. 15):213-228. Abstract available at

Raloff, J. 2003. Second cancer type linked to shift work. Science News 164(July 5):13. Available to subscribers at

______. 2001. Cancer risk linked to night shifts. Science News 160(Nov. 17):317. Available to subscribers at

______. 1998. Does light have a dark side? Nighttime illumination might elevate cancer risk. Science News154(Oct. 17):248. Available at

Schernhammer, E.S., et al. 2006. Night work and risk of breast cancer. Epidemiology 17(January):108-111. Abstract available at

Zhu, Y. . . . R.G. Stevens, et al. In press. Does “clock” matter in prostate cancer? Cancer Epidemiology Biomarkers and Prevention.


David E. Blask
Laboratory of Chrono-Neuroendrocrine Oncology
Bassett Research Institute
Mary Imogene Bassett Hospital
Cooperstown, NY 13326

George C. Brainard
Department of Neurology

900 Walnut Street, Suite 200

Thomas Jefferson University
Philadelphia, PA 19107

Samuel Cos
Department of Physiology and Pharmacology
School of Medicine
University of Cantabria
39011 Santander

Scott Davis
Fred Hutchinson Cancer Research Center
Box 358080, M4-BB74
1100 Fairview Ave. North, Building M
P.O. 19024, Mailstop: M4-BB74
Seattle, WA 98109-1024

William Hrushesky
Dorn Veterans Affairs Medical Center

6439 Garners Ferry Road

Columbia, SC 29209

Mark Rea
Lighting Research Center
Rensselaer Polytechnic Institute

21 Union Street
Troy, NY 12180

Russel J. Reiter
Department of Cellular and Structural Biology
University of Texas Health Science Center
MED-Room 2.058V
San Antonio, TX 78229

Eva S. Schernhammer
Channing Laboratory
Department of Medicine
Brigham and Women’s
Harvard Medical School

Longwood Avenue
Boston, MA 02115

Richard G. Stevens
Department of Community Medicine
University of Connecticut Health Center
263 Farmington Avenue
Farmington, CT 06030

The Light-Cancer Connection

The Light-Cancer Connection

Prevention Magazine

January 2006

Habits like watching late-night TV and reading in bed are linked to cancer. Here’s how to protect yourself.

by Catherine Guthrie

From 1992 to 1999, Eva Schernhammer, MD, worked rotating night shifts in a cancer ward in Vienna, Austria. Her shifts included 10 all-nighters a month in addition to her regular hours; she labored under banks of flickering fluorescent lights through the darkest hours. “I didn’t think much of it,” she says, “until two of my colleagues developed cancer. These were healthy women in their 30s. They really had no risk factors, no family history.” She couldn’t help wondering: Could working late nights be linked to cancer?

Three years later, Schernhammer landed at Harvard Medical School in the Channing Laboratory–the perfect place to find an answer to that question. The lab is home to the Nurses’ Health Study, one of the largest data banks of women’s health information ever amassed. She tapped into medical, work, and lifestyle records gleaned from 78,562 nurses. The end result, published in 2001, was startling: Nurses who’d worked 30 or more years on night shifts had a 36% higher rate of breast cancer, compared with those who’d worked exclusively days.

Three years after her initial study made headlines, she followed up by methodically reviewing 13 new trials on night-shift workers and cancer risk. These findings, published last September, were even more jaw-dropping than her original work: On average, her peers had uncovered a 48% rise in breast cancer among women on the night shift.

Schernhammer had discovered something other researchers had suspected for some time: Exposure to light at night appears to raise the risk of several types of cancer. And evidence suggests that night-shift work isn’t the only risk. Watching TV, sitting in front of the computer, or even reading under a lamp into the wee hours may be enough to throw a wrench into the body’s cancer-fighting machinery.

Simply put, light at night snuffs out one of the body’s most powerful anticancer crusaders, a hormone called melatonin. Because it’s only active at night, melatonin has been dubbed “the hormone of darkness.” It’s the perfect name for a biological superhero. But if melatonin is the body’s superhero, then light is its nemesis, and research suggests it’s gaining the upper hand.

A Case of Bad Timing
“Humans evolved on a planet without electric light over thousands and thousands of generations,” says George Brainard, PhD, a professor of neurology at Thomas Jefferson University who has spent 20 years studying how light affects human biology. “The body is designed to be awake and alert during daytime hours and to sleep at night. Now we have a 24-7 society that isn’t in harmony with our biological design.”

That design is the circadian system–internal rhythms that repeat roughly every 24 hours. The sleep/wake cycle is the rhythm we’re most familiar with, but the body is constantly adjusting its internal machinery–the ebb and flow of hormones, the rise and fall of body temperature, and other subtle rhythms–to mesh with the 24-hour solar day. Monitoring and directing this dance is the brain’s master clock, the suprachiasmatic nucleus (SCN).

The SCN sits directly above the brain stem and houses nearly 20,000 nerve cells that receive and transmit information nonstop to various parts of the brain and body. The signal to which the SCN is most attuned–given its perch above a busy intersection of optic nerves–is light. When wavelengths of light, either from the sun or a bulb, hit the photoreceptors in the back of the eye, signals are sent to the SCN. The SCN resets itself daily by tracking the strength of the signals. Weaker ones at night, when it’s dark, trigger the release of melatonin from the pea-size dollop of brain tissue called the pineal gland.

Acting as the body’s sandman, melatonin slows body functions and lowers blood pressure; consequently, core body temperature drops. Conversely, a strong light signal during the day reverses the process–cortisol and other hormones are released, raising blood pressure and core body temperature, as melatonin production declines.

The Cancer Connection
Cells throughout the body–even cancer cells–have melatonin receptors, and when melatonin makes its nightly rounds, cell division slows. Research by Steven Hill, PhD, a professor of structural and cellular biology at Tulane University, suggests that when this hormone latches on to a breast cancer cell, it counteracts estrogen’s tendency to spur cell growth. In fact, melatonin has a calming effect on several reproductive hormones, male and female, which may explain why it seems to protect against sex hormone-driven cancers (ovarian, endometrial, breast, and testicular).

However, there is one system in the body that resists melatonin’s lulling effect: immunity. Studies have found that the hormone boosts the production of immune cells such as interleukin-2; they help recognize and attack the mutated cells that can lead to cancer. By slowing cancer growth and promoting immunity, melatonin seems designed to offer real protection.

One of the most compelling measures of light’s power to affect cancer rates are studies of the only segment of the population unaffected by light: the blind. Their circadian rhythm ticks on undisturbed by the sun or indoor lamps, and it keeps steady time. Since 1991, four different studies have served up the same results: Blind women have a 50% reduced risk of breast cancer, compared with their sighted peers.

In an attempt to gauge melatonin’s potency, David Blask, MD, PhD, a neuroendocrinologist at Bassett Research Institute in Cooperstown, NY, has been running studies on melatonin, cancer, and light’s effect on both for 28 years. In 2001, he published results from research in which he grafted human breast cancer tumors onto rats, and then divided the animals into two groups. One group was exposed to bright light for 24 hours; the other was exposed to 12 hours of light followed by 12 hours of darkness. In the end, the tumors on rats that were bathed in continuous light grew up to seven times faster than tumors on the rats that spent their nights submerged in total darkness. Blask describes the exposure to bright light at night as comparable to shutting down melatonin production entirely. In people, he says, “melatonin directly inhibits cancer growth. Lose your melatonin signal at night, and you’re losing an important layer of cancer protection.”

Energized by these findings, Blask and Brainard teamed up to take the research one step further. In a just-completed study funded by the National Cancer Institute, they infused human breast cancer tumors with blood taken from healthy, premenopausal women at several times during the day, most notably the predawn hours, when melatonin peaks, and again at night after the subjects had been exposed to bright fluorescent lights for 2 hours. As in the first study, light suppressed melatonin–and tumor growth accelerated. But when melatonin levels peaked at night, tumor growth slowed by 80%. “This is the first proof that light is indeed a risk factor for cancer and the first evidence of a biological link between circadian clock disruption and cancer,” says Blask. “Light at night could be a major driver of breast cancer.”

The studies are scary–and well-established. But not every cancer specialist draws the same conclusions. “There are a lot of hormones in the blood,” says breast oncologist Marisa Weiss, MD, who is president and founder of the Web-based nonprofit “The dance and interplay of hormones has an effect on cancer, but it’s impossible to gauge the influence of just one–especially in studies performed on animals and in petri dishes.” Schernhammer herself is cautious about the research. “I feel more confident saying that an association exists between melatonin, light, and cancer–but not a direct cause and effect. I don’t know what the causes are–maybe factors we haven’t thought of yet.”

The first step in getting a firm answer will be figuring out what kind of cancers may be affected by light. So far, researchers have looked mostly at breast cancer rates in studies of melatonin. But experts suspect that many types of cancer are sensitive to the hormone to some degree. In a 2003 review of women in the Nurses’ Health Study, Schernhammer found a 35% increase in colon cancer rates among night-shift workers. “I think from the beginning, the focus has been on breast cancer,” says Schernhammer, “but by no means is breast cancer the only one affected by melatonin–other cancers, beyond those linked to male or female hormones, show likely links.”

Blask also suspects that melatonin’s protection may extend to other forms of cancer. He has plans to study melatonin suppression in prostate cancer cells, based on earlier test-tube studies that hint at a connection.

Which Lights Are Worst?
Back in 1980, scientists at the National Institute of Mental Health discovered that 2 hours of bright light at night could knock down melatonin to daytime levels, a finding that led them to test bright light as a treatment for the winter-related depression known as seasonal affective disorder. (They had no idea there might be cancer ramifications.) The light they used was intense and continuous, which left Brainard wondering what ordinary household lighting’s effects on melatonin might be. He and his colleagues spent 5 years conducting more than 600 experiments on light and the hormone.

Ultimately, they discovered that short light wavelengths, such as those at the blue end of the spectrum (found in fluorescent and halogen lights), suppressed melatonin the most. On the flip side, long wavelengths, such as those put out by a red lightbulb, stifled melatonin the least. “In a healthy person, you can measurably curb melatonin with as little as 1.3 lux of blue light,” says Brainard. (One lux is the illumination cast by a wax candle.) A 40-watt bulb–what you’d find in a typical reading lamp–is about 50 lux. A moderately lit bedroom measures around 100. Brainard is still working out what happens to melatonin in the face of that kind of exposure, but other research is beginning to suggest answers.

Back in 2001, researchers at the Fred Hutchinson Cancer Research Center in Seattle queried 1,600 women about their sleeping habits over the past 10 years and how brightly lit their bedrooms were. The women who tended to stay up late 2 or 3 nights a week on average had a 70% increase in risk of breast cancer. (Those with bright bedrooms had a barely perceptible increase.)

In one of Schernhammer’s most recent studies, she analyzed morning urine samples taken from women in the Nurses’ Health Study to gauge their nighttime production of melatonin. Women who had the lowest levels also had a 70% higher risk of breast cancer than women with the most melatonin. What surprised her was that the high-risk group wasn’t made up solely of night-shift workers: Women who had worked only days were in the group, as well. “Who knows what these women were doing at night?” she says. But if they were up late for some reason, she speculates, their light exposure may have been enough to chase away melatonin.

Blask’s goal is to further study household lighting’s impact. He has found that breast cancer cells divide faster when light cuts melatonin levels by as little as 15%. His next step is to find out the light intensity and length of exposure needed to trigger cancer cell activity in women’s breasts. Until more is known, one of the best things people can do is protect themselves from unnecessary exposure to light at night, he says.

That advice seems perfectly reasonable to epidemiologist Heather Spencer Feigelson, PhD, a spokesperson for the American Cancer Society. She’s intrigued by the biological mechanism Blask, Brainard, and others have uncovered. “Based on the research, limiting light exposure at night could match the protection from breast cancer that regular physical activity provides,” she says. “And there’s certainly no downside to sleeping in a dark room, aside from bruised shins, maybe.” (For advice on keeping your circadian rhythm on track, see “6 Ways to Reduce Your Risk.”)

Based on Schernhammer’s work, the group at highest risk are people working rotating night shifts under bright lights. She believes the irregularity of the schedule prevents the body from adjusting and adapting; that, in turn, keeps circadian rhythms off track and melatonin production down. Regular night-shift work doesn’t seem as risky as, say, three all-nighters a month, she says.

Should TV and computer screens prove to be culprits in cancer, that may solve, at least in part, the maddening riddle of breast cancer. Last year, 211,000 women were diagnosed with breast cancer and another 40,000 died, yet doctors still have few clues as to what drives the disease. “Even if lighting is at the root of only 10% of breast cancer cases,” says Brainard, “what we learn may help thousands and thousands of women.”

6 Ways to Reduce Your Risk
Researchers are still deciphering how much light at night is too much. But keeping your circadian rhythm running smoothly jibes with healthy living.

Embrace Darkness
Sleep in a completely dark room. Turn off all hall and night-lights. If the glow from street lamps or commercial signs invades your bedroom, consider investing in room-darkening shades or blinds.

Shoot for Nine
A new study of 7,396 Finnish women found that those who slept an average of 9 or more hours a night were one-third as likely to get breast cancer as those who slept 7 to 8 hours. “This is the first evidence of a relationship between sleep duration and breast cancer risk,” says one of the study’s authors, Richard Stevens, PhD, of the University of Connecticut, Farmington.

Choose a Dim Bulb
If you wake up in the middle of the night and can’t sleep, don’t turn on a bright light and read. Just 10 minutes of light is enough to suppress melatonin in some people. Instead, lie in bed to see if you can drift back to sleep. If you must get up, keep the lights dim.

See Red
If you get up in the middle of the night to go to the bathroom, resist the urge to flip on the fluorescent vanity light, says Stevens. Instead, keep a red lightbulb in one fixture. Blue wavelengths cast by fluorescent and halogen lights are the worst offenders. Red wavelengths, on the other hand, are the most innocuous.

Get Some Sun…
The circadian system needs bright light to reset itself. Ten to fifteen minutes of morning sunlight will send a strong timekeeping signal to the brain’s clock, leaving it less likely to be confused by weaker signals from night-lights, says Mariana Figueiro, PhD, a researcher at the Lighting Research Center in Troy, NY. If it’s dark when you leave for work or dusk when you get home, a brisk lunchtime walk will help.

And Get More Sun As You Age
In older adults, the brain’s master clock, the suprachiasmatic nucleus (SCN), shrinks and slows down, which makes the receptors less alert to light signals. “Due to eye changes, a 60-year-old gets one-third the amount of light that a 20-year-old gets,” says Figueiro. If you can’t be outdoors as much as you’d like, throw open the blinds and position a chair near a window to maximize rays.

What about supplements?
Melatonin pills are readily available, but experts don’t recommend taking them. “It’s a very powerful hormone,” says Blask. “We just don’t know if it’s safe to take.” However, research on cancer patients suggests that melatonin can boost survival and quality of life for people with tough-to-treat cancers like lung cancer and melanoma. And taken alongside chemotherapy, it seems to improve appetite and ease other side effects. But check with your doctor before adding melatonin to your regimen.

New Research Shows Artificial Light At Night Stimulates Breast Cancer

New Research Shows Artificial Light At Night Stimulates Breast Cancer

CONTACT: John Peterson, 919-541-7860, [email protected]


Results from a new study in laboratory mice show that nighttime exposure
to artificial light stimulated the growth of human breast tumors by
suppressing the levels of a key hormone called melatonin. The study also
showed that extended periods of nighttime darkness greatly slowed the
growth of these tumors.

The study results might explain why female night shift workers have a
higher rate of breast cancer. It also offers a promising new explanation
for the epidemic rise in breast cancer incidence in industrialized
countries like the United States.

The National Cancer Institute and the National Institute of
Environmental Health Sciences, agencies of the federal National
Institutes of Health, provided funding to researchers at the Bassett
Research Institute of the Mary Imogene Bassett Hospital in Cooperstown,
New York and The Thomas Jefferson University in Philadelphia, Pa. The
results are published in the December 1, 2005 issue of the scientific
journal “Cancer Research”.

“This is the first experimental evidence that artificial light plays an
integral role in the growth of human breast cancer,” said NIEHS Director
David A. Schwartz, M.D. “This finding will enable scientists to develop
new strategies for evaluating the effects of light and other
environmental factors on cancer growth.”

“The risk of developing breast cancer is about five times higher in
industrialized nations than it is in underdeveloped countries,” said Les
Reinlib, Ph.D., a program administrator with the NIEHS’ grants division.
“These results suggest that the increasing nighttime use of electric
lighting, both at home and in the workplace, may be a significant

Previous research showed that artificial light suppresses the brain’s
production of melatonin, a hormone that helps to regulate a person’s
sleeping and waking cycles. The new study shows that melatonin also
plays a key role in the development of cancerous tumors.

“We know that many tumors are largely dependent on a nutrient called
linoleic acid, an essential fatty acid, in order to grow,” said David
Blask, M.D., Ph.D., a neuroendocrinologist with the Bassett Research
Institute and lead author on the study. “Melatonin interferes with the
tumor’s ability to use linoleic acid as a growth signal, which causes
tumor metabolism and growth activity to shut down.”

To test this hypothesis, the researchers injected human breast cancer
cells into laboratory mice. Once these cells developed into cancerous
tumors, the tumors were implanted into female rats where they could
continue to grow and develop.

The researchers then took blood samples from 12 healthy, premenopausal
volunteers. The samples were collected under three different conditions
— during the daytime, during the nighttime following 2 hours of
complete darkness, and during the nighttime following 90 minutes of
exposure to bright fluorescent light. These blood samples were then
pumped directly through the developing tumors.

“The melatonin-rich blood collected from subjects while in total
darkness severely slowed the growth of the tumors. “These results are
due to a direct effect of the melatonin on the cancer cells,” said
Blask. “The melatonin is clearly suppressing tumor development and

In contrast, tests with the melatonin-depleted blood from light-exposed
subjects stimulated tumor growth. “We observed rapid growth comparable
to that seen with administration of daytime blood samples, when tumor
activity is particularly high,” Blask said.

According to the researchers, melatonin exerts a strong influence on the
body’s circadian rhythm, an internal biological clock that regulates
sleep — wake cycle, body temperature, endocrine functions, and a number
of disease processes including heart attack, stroke and asthma.
“Evidence is emerging that disruption of one’s circadian clock is
associated with cancer in humans, and that interference with internal
timekeeping can tip the balance in favor of tumor development,” said

“The effects we are seeing are of greatest concern to people who
routinely stay in a lighted environment during times when they would
prefer to be sleeping,” said Mark Rollag, Ph.D., a visiting research
scientist at the University of Virginia and one of the study co-authors.
“This is because melatonin concentrations are not elevated during a
person’s normal waking hours.”

“If the link between light exposure and cancer risk can be confirmed, it
could have an immediate impact on the production and use of artificial
lighting in this country,” said Richard Stevens, Ph.D., an
epidemiologist with the University of Connecticut Health Center who has
authored several papers on the subject. “This might include lighting
with a wavelength and intensity that does not disrupt melatonin levels
and internal timekeeping.”

“Day workers who spend their time indoors would benefit from lighting
that better mimics sunlight,” added Stevens. “Companies that employ
shift workers could introduce lighting that allows the workers to see
without disrupting their circadian and melatonin rhythms.”

NIEHS, a component of the National Institutes of Health, supports
research to understand the effects of the environment on human health.
For more information on breast cancer and other environmental health
topics, visit our website at

The National Institutes of Health (NIH) — “The Nation’s Medical
Research Agency” — includes 27 Institutes and Centers and is a
component of the U. S. Department of Health and Human Services. It is
the primary Federal agency for conducting and supporting basic,
clinical, and translational medical research, and it investigates the
causes, treatments, and cures for both common and rare diseases. For
more information about NIH and its programs, visit

Study Demonstrates Role Of Exercise In Modifying Melatonin Levels

Study demonstrates role of exercise in modifying melatonin levels

Increase believed to offer breast cancer protection

Toronto, ON – December 1, 2005: Moderate physical activity, which is believed to help reduce the risk of breast cancer, may do so because it increases production of a hormone believed to have protective effects against the disease, a Canadian research team has learned.

Researchers at Mount Sinai Hospital’s Samuel Lunenfeld Research Institute in Toronto have completed a study of how light and other factors, such as physical activity, influence the production of melatonin – a hormone released mainly at night in the absence of light and believed to protect against breast cancer. The findings of the study have been published in the December 1, 2005 edition of the American Journal of Epidemiology.

The study is among the first to demonstrate the role of physical activity in potentially modifying melatonin levels, thus creating biological evidence for a reduced risk of breast cancer among women who exercise regularly.

“If we know that increased melatonin production could be an underlying reason for the protective effects of physical activity against breast cancer, then there is great opportunity for the scientific community to build on this knowledge and help women understand what steps they can take to reduce their risk of developing the disease,” said Dr. Julia Knight, the study’s lead investigator at the Samuel Lunenfeld Research Institute at Mount Sinai Hospital and a University of Toronto professor in the Department of Public Health Sciences.

The study, funded through the Canadian Breast Cancer Research Alliance with special funding from the Canadian Breast Cancer Foundation – Ontario Chapter to study the modifiable risk factors for breast cancer, took place between 2002 and 2004 and involved the participation of 213 female volunteers whose melatonin levels were monitored over time.

“Our focus on primary prevention research was initiated with the long-term goal of building knowledge about how to stop breast cancer before it starts,” said Sharon Wood, Executive Director of the Canadian Breast Cancer Foundation – Ontario Chapter. “We are encouraged by the findings of Dr. Knight’s work, which could provide momentum for lifestyle changes that could lead to a healthier population and potentially reduce the incidence of breast cancer.”

Melatonin, Sleep Aid That May Fight Cancer

Melatonin, sleep aid that may fight cancer

By Judy Foreman The Boston Globe

Melatonin, long known to insomniacs as an over-the-counter sleep aid, is now being studied as a way to prevent and treat breast and other cancers.


Dubbed the “hormone of darkness,” melatonin is made by the brain’s pineal gland at night. This summer, researchers at Brigham and Women’s Hospital in Boston led by Dr. Eva Schernhammer, an epidemiologist, showed that women who produced the lowest levels of melatonin were 70 percent more likely to get breast cancer than those with the highest levels.


Schernhammer’s group previously showed that women who work at night are at higher risk of both breast and colon cancer. Light at night can shut off melatonin production.


A study to be published this autumn explores whether women who sleep nine hours or more a night – enabling them to produce more than the average amount of melatonin – are at lower than average risk of breast cancer.


A co-author of that study, the cancer epidemiologist Richard Stevens of the University of Connecticut Health Center in Farmington, said breast cancer rates were much higher in industrialized countries, where, among other things, people routinely use a lot of artificial light at night, which suppresses melatonin production.


“We can’t say yet, but the evidence is accumulating that light at night, and the consequent decrease in melatonin, may be a major driver of breast cancer,” he said.


From an evolutionary point of view, melatonin may have developed as a signal to tell animals when to breed. In sheep, melatonin levels rise in autumn as the nights get longer.


Melatonin is also an important regulator of the circadian clock in the brain, which keeps the body on a regular cycle of day and night. Light, whether from the sun or electric lights, suppresses melatonin production. But when light disappears, and darkness falls, there’s a cascade of nerve signals from the eye to the pineal gland, which then starts making melatonin. That’s why melatonin has been popularized as a sleep aid.


Frustrated by the high rates of breast cancer in industrialized countries, Stevens of UConn hypothesized in the late 1980s that light at night might spur cancer growth and that melatonin might protect against it.


“We know that if you take out the pineal gland in animals, that removes all melatonin, and then if you inject cancer cells, the cancer growth rate increases,” said Steven Lockley, a neuroscientist at Brigham and Women’s, who is now studying the melatonin levels and breast cancer rates of women who are blind. “We know that when you put an animal in constant light, that also stops all melatonin production, and you get a similar response. And if you then treat an animal with melatonin, you can slow down the cancer rate.”


Researchers are just now starting to look at the treatment potential for melatonin. At the Bassett Research Institute in Cooperstown, New York, Dr. David Blask, a senior research scientist, reported at a cancer meeting this summer that melatonin can “put cancer cells to sleep” by blocking their ability to soak up linoleic acid, which makes cancer cells grow rapidly. In animal studies, Blask said he had found that cancer cell growth is slower at night, when melatonin is highest, and faster during the day. He also found that adding melatonin to human breast cancer cells grown in rats can slow the cancer’s growth.


In Europe, studies of people with cancer who are given melatonin are also promising, though preliminary. Melatonin appears able not only to slow cancer progression and improve survival in advanced cancer patients, but to protect healthy cells from the side effects of chemotherapy and radiation, said Dr. Fade Mahmoud of the University of South Dakota School of Medicine, who published a review of the studies this summer.


Italian researchers, in a long series of human studies, have shown that melatonin, which appears to have little toxicity, can boost survival at least modestly in some people with melanoma and cancers of the lung, breast, kidney and other organs.


While it’s too soon to rush out and buy over-the-counter melatonin to fight cancer, it is a good idea to “live a melatonin-friendly lifestyle,” said Stevens of UConn.


That means going to bed earlier if you’re a night owl, making sure the bedroom is dark, and keeping the light dim in the bathroom if you make nightly trips there.

Advancing the Effective Use of Light for Society

Advancing the effective use of light for society 

Wednesday, January 12, 2005

How Does Light at Night Affect Your Health?

LRC presents findings on the effects of light at night LRC researchers are updating the scientific, medical, and lighting communities regarding ongoing research in the area of light and health. The LRC’s continuing study of light’s impact on human and mouse circadian systems may help medical researchers to reassess important health issues such as potential cancer risks from light at night. This work includes exploring the implications of lighting from various light sources for different populations, including teens with delayed sleep phase syndrome, seniors with Alzheimer’s
disease, premature infants in neonatal intensive care units, persons with seasonal affective disorder, and nurses who work day and night shifts.

Circadian responses to light, potential implications for

Light at night can shift a body’s circadian rhythms, which regulate the sleep/wake cycle. However, light at night has other effects as well, including suppressing the production of the hormone melatonin. Because melatonin is an anticancer agent, light at night may actually contribute to increases in cancer cell growth. Therefore, understanding the characteristics of light that suppress melatonin production in humans is extremely important.

To disseminate the latest information on the biological effects of light, LRC researchers presented their findings on the biological effects of light and related subjects at several conferences during the fall of 2004.

In Vienna, Austria, Dr. Mariana Figueiro, head of the LRC’s Light and Health program, and LRC Director Dr. Mark Rea attended the Commission Internationale de l’clairage (CIE) conference, “Expert Symposium on Light and Health.” They provided new data supporting the phenomenon of spectral opponency in melatonin suppression. Their paper, co-written with LRC researchers Dr. John Bullough and Andrew Bierman, is called, “Spectral opponency in human circadian phototransduction: implications for lighting practice.”

Although melatonin is suppressed most by short-wavelength (blue) light, LRC studies suggest that when blue light is combined with longer wavelengths (to make white light), it becomes less effective as a stimulus to the circadian system. “Apparently the circadian system shares neural circuits with color mechanisms in the eye,” says Rea. (Read the study’s details in the April 2004 LRC newsletter.)

At the “Cancer and Rhythm” conference in Graz, Austria, LRC researchers advocated caution in interpreting research on rodent species with respect to light at night and cancer. At the conference, Rea and Figueiro presented a poster on this subject called, “Linking light at night to cancer risk in humans: does the evidence add up?” co-authored by Andrew Bierman and Terry Klein.

Most rodents are nocturnal and highly sensitive to light, according to Dr. Bullough, who studied lighting’s effect on mice. “A comparison of light levels needed to impact the circadian system shows that only very low levels of light are needed to impact a mouse’s circadian system,” says Bullough. “But very high levels are necessary to impact a human’s circadian system” more than 1,000 times the levels mice need.” LRC research also indicates that the circadian system of mice does not exhibit spectral opponency, so
rodents should be relatively more sensitive to white light than humans.

Measuring light’s impact on the circadian system

As part of their study into the impact of light on the human circadian system, LRC researchers also have developed a unique light-measuring tool called the Daysimeter. The Daysimeter accurately characterizes spectral sensitivity to short-wavelength (blue) light; can incorporate spectral opponency; and can measure retinal light exposure rather than light on a desktop or other visual tasks. (Read more about the Daysimeter in the July 2004 LRC Newsletter.) The
project was sponsored by the Daylight Dividends program.

Rea spoke about the Daysimeter at “Emerging Topics in Breast Cancer and the Environment Research,” a conference held in Princeton, N.J. Dr. Rea presented an informative poster co-authored with Bierman and Klein titled “Of mice and women: Light at night and cancer risk,” describing the light-measuring characteristics of the Daysimeter. He received funding for his travel from the Susan G. Komen Breast Cancer Foundation.

About the LRC
The Lighting Research Center (LRC) is part of Rensselaer Polytechnic Institute and is the leading university-based research center devoted to lighting. Founded in 1988, the Lighting Research Center has built an international reputation as a trusted and reliable source for objective information about lighting technologies, applications, and products. Its mission is to advance the effective use of light and create a positive legacy of change for society and
the environment.

© 2005 Rensselaer Polytechnic Institute, Troy, NY 12180 USA. 

Sleeping with Lights on Might Trigger Cancer Through Melatonin Suppression

Sleeping with lights on might trigger cancer through melatonin suppression

Richard Woodman
Reuters Health
Posting Date: September 8, 2004

Last Modified: November 1, 2001
Last Updated: 2004-09-08 9:01:01 -0400 (Reuters Health)

LONDON (Agence de Presse Medicale for Reuters Health) – Parents were advised on Wednesday not to allow children to sleep with the lights on because of the theoretical possibility that too much light at night could suppress levels of melatonin.

Russel Reiter, Professor of Cellular and Structural Biology at the University of Texas, said this was important because a reduction in melatonin had been linked to cancer initiation and cancer progression, he told the Children with Leukaemia conference in London.

“As an antioxidant, in many studies melatonin has been shown to protect DNA from oxidative damage. Once damaged, DNA may mutate and carcinogenesis may occur,” he said.

“Likewise, lower than normal melatonin levels may exaggerate the growth of tumours since melatonin inhibits the uptake of fatty acid growth factors by cancer cells, inhibits telomerase activity and endothelin-1 synthesis…”

Reiter said night workers were known to have a higher risk of breast cancer. Studies also showed that blind people, who are not vulnerable to reduced melatonin levels through light at night, had a lower incidence of cancer.

He told APM that while there was still no proof that suppression of melatonin by light caused cancer, some precautionary measures should be taken. “There are parents who allow children to sleep with the lights on. This should be absolutely prohibited,” he said.

Copyright 1994-2004 © Trustees of the University of Pennsylvania 

Hormone Melatonin Slows Breast Cancer - Bright Light at Night Linked to Increased Cancer Risk

Hormone Melatonin Slows Breast Cancer – Bright Light at Night Linked to Increased Cancer Risk

By Daniel DeNoon
WebMD Medical News

July 14, 2003 (Washington, D.C.) — The nighttime hormone melatonin puts breast cancer cells to sleep. It also slows breast cancer growth by 70%.

David E. Blask, MD, PhD, of Bassett Research Institute in Cooperstown, N.Y., reported the findings at this week’s annual meeting of the American Association for Cancer Research.

Breast cancers get revved up by a kind of dietary fat called linoleic acid. Melatonin interacts with linoleic acid, so he gave melatonin to mice implanted with human breast cancers.

“This breast cancer rev-up mechanism gets revved down by melatonin,” Blask said at a news conference. “Nighttime melatonin is a relevant anticancer signal to human breast cancers. Ninety percent of human breast cancers have specific receptors for this signal.”

The hormone seeps from a pea-sized gland in the brain when the lights go out at night. It’s the reason you get sleepy when it’s dark. Blask and colleagues found that melatonin puts cancer cells to sleep, too. 

Too Much Night Light May Be Boosting Cancer Rates

Too much night light may be boosting cancer rates

Exposure to excessive artificial light at night may be disrupting the body’s natural melatonin balance and raising the incidence of some cancers, American researchers have proposed.

Professor George Brainard, a neuroscientist at Philadelphia’s Thomas Jefferson University, reported early results of his research at the annual meeting of the American Association for the Advancement of Science in Denver, USA.

He also announced the discovery of what appears to be a novel ‘photoreceptor system’ in the human eye that regulates the biological and behavioural effects of light on the body. His team has established that specific wavelengths control the production of melatonin, with the blue region of the visible spectrum being the most potent.

He said that a growing body of research suggests that too much exposure to light at night can interrupt the body’s production of melatonin, which has been shown in the laboratory to inhibit the growth of some types of cancer cells. Light-induced melatonin suppression has also been shown to increase tumour growth in animal models.

Melatonin is produced by the pineal gland in the brain, and is an essential part of the system that controls the body’s circadian (or day-night) rhythm. The eye uses the presence of light to regulate the production of melatonin; the body then produces high levels of melatonin at night, and low levels during the day.

The increased and unnatural levels of light we are exposed to in the modern world after dark (and the ensuing disruption in the production of melatonin) may be raising the risk – as well as the incidence – of cancers such as breast cancer, argued Brainard.

The researcher, who has spent much of the past two decades studying how the brain interprets and reacts to light, backed up this hypothesis by citing the higher incidence of breast cancer in wealthy countries – where people are exposed to more artificial light at night – compared with the much lower incidence in developing countries, where exposure to light at night is less significant. 

Light at Night Risk for Breast Cancer

Light at Night Risk for Breast Cancer

Ivanhoe Newswire

Researchers found light at night may be associated with an increased risk in breast cancer. Richard G. Stevens, Ph.D., from the University of Connecticut Health Center, presented his findings this week at the “Era of Hope” Department of Defense Breast Cancer Research Program meeting in Orlando, Fla.

Stevens says electromagnetic fields released by light suppress the normal nocturnal production of melatonin, which could increase the release of estrogen by the ovaries. He says this would explain why industrialized societies have five times the risk of breast cancer.

Stevens and his colleagues examined the association between light and breast cancer risk in 800 healthy women and 800 women with breast cancer in Seattle. Researchers gathered information on sleep habits and bedroom lighting in the 10 years before diagnosis and lifetime occupational history. Results show a slight increase in risk for women who frequently did not sleep during the period of the night when melatonin levels are typically at their highest. There was an indication of increased risk among women with the brightest bedrooms. Graveyard shiftwork was also associated with increased breast cancer risk, with a trend of increased risk with increasing years and with more hours per week of graveyard shiftwork.

Previous studies show slight increases of breast cancer risk for flight attendants and rotational shift nurses, but half the risk for blind women. Stevens suggests researchers also examine restaurant workers and women who work in photography dark rooms. In addition, he says researchers should study light effects on mammary tissue in rats. Though several studies show low or no associations between electromagnetic fields and breast cancer risk, Stevens and fellow panelists at the meeting agree there is not a lot of concrete evidence and many questions remain. 

Symposium 2000: Low frequency EMF, Visible Light, Melatonin and Cancer

Symposium 2000: Low frequency EMF, Visible Light, Melatonin and Cancer

Summaries of proceedings
Summary of International Symposium Entitled ’Low Frequency EMF, VisibleLight, Melatonin and Cancer’

Reiter RJ. Department of Cellular and Structural Biology, The University of Texas Health Science Center; San Antonio; Texas USA. E-Mail: [email protected]

The potential relationship of melatonin and cancer has been widely discussed in recent years. Since melatonin is a known oncostatic agent, any factor that depresses its production, secretion or actions
in humans may contribute to an increased cancer risk. This was the basis of this symposium inasmuch as light exposure at night unequivocally reduces the nocturnal rise in melatonin levels in
the blood. Furthermore, exposure to extremely low frequency electromagnetic fields (EMF) has reported effects on circulating melatonin levels. The EMFs of interest were those to which humans
are exposed near transmission lines, electrical appliances, etc. The meeting in Cologne brought together experts from around the world to discuss the state of knowledge in these fields. Clearly, the incidence of at least certain types of cancer, e.g., breast cancer, has increased substantially with increased industrialization. The implication is that this rise in cancer is somehow related to economic development. One feature that is commonplace throughout the world today is the use of electricity. Indeed, the consumption of electric power increased several 1000-fold during the 20th century. With this increased use there is obviously an increased exposure to light at night (when it would normally be dark) and augmented exposure to EMF. About 10 years ago it was proposed that the rise in cancer incidence may be a consequence of a generalized reduction in

Does Light Have a Dark Side? Nighttime Illumination Might Elevate Cancer Risk

Does Light Have a Dark Side? Nighttime illumination might elevate cancer risk

By J. Raloff

Since life began, one pattern has dominated Earth’s natural environment—a daily rhythm of intense sunlight alternating with nights of near-total darkness. As a source of heat and energy, sunlight powers a majority of the planet’s biological activities. When that light disappears, much of the world rests.

Humans, the grand manipulators, have not been content to cede control of their activity cycle to the heavens, however. People have spent eons developing ever better means to artificially extend the day. Thanks to widespread electrification and color-corrected, high-watt lightbulbs, synthetic sunlight can now bombard city dwellers around-the-clock.

This attempt to erase the night—or at least to confine it to small, artificially defined windows—may come with a price. At a minimum, it can lead to a chronic lack of sleep, diminishing the effectiveness of the body’s immune system. Some new studies, however, suggest the possibility of an even more worrisome threat.

Exposure to light at night can disrupt the body’s production of melatonin, a brain hormone best known for its daily role in resetting the body’s biological clock (SN: 5/13/95, p. 300). Secreted primarily in the brain, and at night, melatonin triggers a host of biochemical activities, including a nocturnal reduction in the body’s production of estrogen. Some researchers have speculated that chronically decreasing nocturnal melatonin production—as with light—might increase an individual’s risk of developing estrogen-related malignancies, such as breast cancer.

Two studies in Nordic populations now offer tentative support for this idea.

According to neuroendocrinologist Russel J. Reiter of the University of Texas Health Science Center at San Antonio, the emerging science indicates that, functionally, “light is a drug”—and that “by abusing it, we risk imperiling our health.”

Light entering the eye allows our brains to sense the shape, size, color, and motion of objects around us. It also summons, albeit imperceptibly, a cadre of other biological sentinels. These go on to trumpet light’s presence to distant tissues—organs and cells lacking the means to detect illumination directly.