Pineal Melatonin & Breast Cancer

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Light at Night

A Lifetime of Working the Night Shift May Increase Breast Cancer Risk

By Melinda T. Willis

Oct. 16 — 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.

Cycles Interrupted

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. s, 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 s.

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 s.

http://abcnews.go.com/sections/living/DailyNews/breastcancer011016.ht

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