Breast cancer risk rises with night-shift hours. (Melatonin Implicated).

Women who work the night shift are at increased risk of developing breast cancer, and the risk rises in a dose-dependent fashion with increasing hours per week and with increasing years spent at shift work, researchers found in two separate studies.

The findings indicate that exposure to light during the nighttime hours, not the night-shift work itself, somehow promotes tumor growth, most likely by suppressing the pineal gland's normal nocturnal production of melatonin, both groups of investigators reported.

In the first study, sleep and work patterns were assessed in 813 women who were newly diagnosed with breast cancer between 1992 and 1995 and in 793 age matched control subjects, according to Scott Davis, Ph.D., of the Fred Hutchinson Cancer Research Center, Seattle, and his associates.

Women who worked the night shift during the 10 years preceding enrollment in the study were found to have a 60% higher risk for breast cancer, compared with those who had never worked the night shift. The risk rose significantly with each hour of such work per week, so that those who worked at least 5.7 hours per week on the night shift had more than twice the risk, compared with those who never worked the night shift.

The case patients worked more hours per week (7.2 hours) than did the control subjects (4.6) and worked more years at jobs requiring night work (4.5 years vs. 3.1 years). Independently of their work-shift status, women who often didn't sleep during the period at night when melatonin levels typically peak showed a 14% increase in breast cancer risk for every night of wakefulness per week.

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