Melatonin Raises Breast Cancer Risk
Exposure to light while
working at night may increase a woman's risk of developing breast
cancer. And, it seems, women in developed countries are worse
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 American 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
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
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.
Reference Source 104