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Colorectal Cancer Health Center

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Night Shifts May Raise Cancer Risk

Study Links Levels of Hormone Melatonin to Cancer Rates
WebMD Health News

June 3, 2003 (Chicago) -- Working night shifts at least three times a month might increase a woman's risk of developing colorectal cancer, according to researchers at Harvard's Brigham and Women's Hospital.

Data from an ongoing study of almost 80,000 nurses already suggest that working nights increases a woman's risk of breast cancer, says lead researcher, Eva S. Schernhammer, MD, instructor in medicine at Harvard, and a new analysis points to a similar association for colorectal cancer. And the explanation for this link may be the lights shinning over night workers' heads, Schernhammer tells WebMD.

The Role of Melatonin

During the dark nighttime hours, the body produces a hormone called melatonin, "which is also called the hormone of the dark," Schernhammer says. "The peak production of melatonin occurs at about 1 or 2 a.m." Exposure to light at night stops the production of melatonin.

In animal experiments and in some laboratory studies, melatonin demonstrated the ability to protect against the development of cancers, and several researchers suggest that it works the same way in humans, says Schernhammer.

But artificial lights suppress melatonin production, "so night workers have lower levels of melatonin, since almost no melatonin is produced during daylight hours," she says. Women who worked occasional night shifts for 15 years or more had a 35% increase in risk for intestinal cancer compared with women who didn't work nights.

In the study, which is published in the June 4 issue of the Journal of the National Cancer Institute, Schernhammer and her colleagues asked 78,586 participants in the Nurses' Health Study to answer questions about shift work. Over a follow-up period from 1988-1999, 602 cases of colorectal cancer were found.

"The question we asked was about how many months they worked three or more night shifts a month," Schernhammer says. "Since we analyzed the data, I've been talking to nurses and I know that most nurses who work night shifts work more than three nights a month -- the usually work seven to eight nights a month."

Should You Quit?

Judy E. Garber, MD, MPH, director of cancer risk and prevention at the Dana-Farber Cancer Institute in Boston, tells WebMD though the study findings are interesting, there is no need for women who work nights to quit their jobs.

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