Second cancer type linked to shift work

Two years ago, a pair of research teams each reported finding an elevated risk of breast cancer among women who sometimes worked nightshifts. One of the teams now finds that those women also face an increased risk of colorectal cancer.

Eva S. Schernhammer of Brigham and Women’s Hospital in Boston and her colleagues pored over data on 600 colorectal cancers diagnosed among 78,500 nurses participating in a long-running health study. As before, the researchers compared cancer incidence in women who reported working the graveyard shift with that of women who never worked nights.

In the June 4 Journal of the National Cancer Institute, the epidemiologists report finding a 35 percent higher risk of colorectal cancer among workers who pulled night duty several times a month for at least 15 years.

Schernhammer’s team suspects this link may trace to a reduced secretion of the hormone melatonin by the brains of people working nights. Melatonin is produced mainly when a person is in darkness. Several recent studies showed that colon cancer patients tend to secrete less melatonin than healthy people do. Animal studies have also indicated that melatonin can inhibit the development of cancer (SN: 10/2/99, p. 221).


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Janet Raloff is the Editor, Digital of Science News Explores, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer, specializing in toxicology. To her never-ending surprise, her daughter became a toxicologist.

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