Night-Shift Work and Risk of Colorectal Cancer Among Nurses

Night shift work and, specifically, exposure to light at night that suppresses the physiologic production of melatonin, has been linked to higher rates of colorectal cancer among nurses.

In a study published in the most recent issue of the Journal of the American Cancer Institute, a group of researchers from Harvard Medical School and Brigham and Women's Hospital led by Dr. Eva S. Schernhammer examined the relationship between working rotating night shifts and the risk of colorectal cancers among female participants in the Nurses' Health Study. They documented 602 incident cases of colorectal cancer among 78,586 women who were followed from 1988 through 1998.

Compared with women who never worked rotating night shifts, women who worked 1 to14 years on rotating night shifts had a higher risk of colorectal cancer, while women who worked 15 years or more on rotating night shifts had a 35 percent greater risk of developing rectal or colon cancer.

"These data suggest that working a rotating night shift at least three nights per month for 15 or more years may increase the risk of colorectal cancer in women," the researchers concluded.

Melatonin is a hormone that has antiproliferative effects on intestinal cancers. Although observational studies have associated night-shift work with an increased risk of breast cancer, the effect of night-shift work on the risk of other cancers is not known.

"Because night-shift work has become very common in developed countries, future studies should assess the relationship of light exposure to the risk of other cancers and consider the risks in men," said the researchers.

"If melatonin's anti-cancer properties are the source of our observed effects, this research opens a whole new arena of potential associations between exposure to light and a variety of cancers," they said, noting further study is needed.

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