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Night Shift May Carry Cancer Risk

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Working the night shift a few nights a month can do more than leave you bleary-eyed, a study suggests today. It just might increase your risk of colorectal cancer.

Nurses who worked the night shift at least three times a month for 15 years or more were 35% more likely to develop that type of cancer than nurses who never worked nights, Harvard University researchers report in the Journal of the National Cancer Institute.

The finding, from the ongoing Nurses Health Study, is the first to link night-shift work with a higher risk of colorectal cancer. The disease is expected to kill 57,100 Americans this year, the American Cancer Society says. Other research, including a 2001 report out of the Nurses Health Study, has suggested that working rotating night shifts raises women's risk of breast cancer.

The researchers asked 78,586 nurses in 1988 whether they had ever worked three or more night shifts a month. The nurses indicated whether they had done so fewer than 15 years or 15 years or more. The scientists followed the women through 1998 to see who developed colorectal cancer.

Researchers suspect lowered levels of melatonin, a hormone involved in regulating sleep, might increase night workers' cancer risk.

Melatonin levels usually peak in the middle of the night. Turn on the lights, though, and they decline measurably in just 10 minutes, says Edward Stevens, a cancer epidemiologist at the University of Connecticut who was not involved in the new study. After two weeks of intermittent nightly exposure to light, humans experience a dramatic reduction in melatonin production, Nurses Health Study researchers write.

In the lab, melatonin appears to inhibit tumor growth. Other studies suggest that colorectal cancer patients have lower blood levels of melatonin than healthy people, the authors write. However, that could be a result of the disease, not a cause of it, Stevens says.

Study co-author Francine Laden, of Boston's Brigham and Women's Hospital, says the findings should spur more research into whether sleeping in as dark a room as possible during the day could counteract night-shift workers' decline in melatonin production.

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