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Women who work nights and those who are up frequently during the early hours of the morning may be at an increased risk for developing breast cancer. The findings, combined with those of other studies in humans and in animals, suggest that nighttime exposure to light may elevate breast cancer risk by suppressing the production of melatonin, a brain hormone that is made during darkness and that normally peaks at night.

The results of this study provide evidence that indicators of exposure to light at night may be associated with the risk of developing breast cancer. Researchers from Harvard Medical School also examined the relationship between breast cancer and working the night shift in more than 78,000 nurses.

Breast cancer risk was higher among women who often did not sleep during the period of the night when levels of the sleep-regulating hormone melatonin are typically at their highest. Their breast cancer risk increased 14% for each night of the week that they reported not sleeping during this period.

Women who worked between one and 29 years on a rotating night-shift schedule had an 8% increase in breast cancer risk, while women who worked 30 or more years on the night shift showed a 36% increase in their risk of developing breast cancer.

Women who worked on rotating night shifts with at least three nights per month, in addition to days and evenings in that month, also appeared to have a moderately increased risk of breast cancer after extended periods of working rotating night shifts. Working the graveyard shift upped a woman's risk of developing breast cancer by 60%.

Nighttime light exposure can come from a variety of sources, including working the night shift, suffering from insomnia or even being exposed to light in the bedroom during sleep. Exposure to light at night, by suppressing melatonin production, may increase women's levels of the sex hormone estrogen, which stimulates growth of breast tissue, including some breast cancers.

Blind women, whose melatonin production does not drop upon exposure to light, have lower breast cancer rates than sighted women. The Journal of the National Cancer Institute (October 17, 2001;93:1513-1515, 1557-1568) added that laboratory studies and experiments in animals suggest that melatonin may directly inhibit the growth of breast cancer cells.