Oct. 16, 2001 -- Exposure to light during the nighttime may increase a woman's risk for developing breast cancer, two studies in this month's Journal of the National Cancer Institute reveal.
Routinely working the night shift is one way that women can be exposed to light at night.
The first study, based on questionnaires from 78,562 women participating in the Nurses' Health Study, reports that women who worked 30 or more years on the night shift, with at least three night shifts per month, had an almost 40 percent greater risk of developing breast cancer compared with those who worked the usual 9-to-5 shift.
An 8 percent increase in breast cancer risk was found in women who worked night shifts for less than 30 years.
The second study reports that nighttime bright light exposure is linked to increased breast cancer risk.
The precise reason why late-shift work increases cancer risk is not well understood, though interference with the body's light-dark hormone cycles seems a likely culprit, the experts say.
"There are a lot of theories," says Dr. Eva Scherenhammer of Brigham and Women's Hospital in Boston and the lead author of the first study.
One theory is that decreased levels of the brain hormone melatonin are responsible, since this chemical is known to regulate daily sleep-wake cycles. Previous research suggests that unusually low levels of melatonin, which can be seen if humans are exposed to light during the night, may promote tumor growth. Normally, melatonin levels are highest during nighttime darkness and lowest during the daytime light.
"Another theory is that while melatonin levels are depressed, female hormones are increased," says Scherenhammer. Lifetime exposure to the female hormone estrogen is an established risk factor for breast cancer.
Putting It Into Perspective
Women who work the night shift for 30 or more years represented only 1.8 percent of Scherenhammer's study population and are not likely to constitute a large percentage of the population in general.
"If you look at this on a population level, the effect may lead to maybe one new breast cancer case per year," says Scherenhammer.
However, the implications of these findings extend beyond women who work at night, says Dr. Richard Stevens, a cancer epidemiologist at the University of Connecticut Health Center and co-author of the second study.
"Women in developing countries have one-fifth the risk of breast cancer compared to women in industrialized nations," says Stevens.
It is possible that exposure to more light at night, a common phenomenon in industrialized nations, may account for increased cancer risk in women. "This has implications that are independent of shift work," adds Stevens.