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Night Shift Work: Risk Factor for Breast Cancer?
Findings of a few studies have suggested that women who frequently work overnight shifts have increased risk for breast cancer. According to one theory, this association is mediated by reductions in serum melatonin that follow nocturnal exposure to light. Melatonin suppresses tumor growth in experimental animal models, and reduced melatonin levels may increase ovarian estrogen release.
In 1988, 78,562 nurses with no history of breast cancer were questioned about previous overnight shift work; interview participants then were followed for 10 years. In an analysis adjusted for potential confounders, women who had worked rotating night shifts for 30 years or longer had significantly increased risk for developing breast cancer (relative risk, 1.36) compared with those who had never worked night shifts. Among nurses with less than 30 years of shift work, risk was slightly elevated (RR, 1.08) but fell just short of significance.
In another study, researchers compared 813 women with breast cancer and 793 age-matched controls. A history of overnight shift work during the previous 10 years was associated with significantly increased risk for breast cancer (odds ratio, 1.6, after adjustment for other risk factors). Breast cancer risk also was increased significantly among women who frequently did not sleep during the middle of the night for any reason (OR, 1.7 for the group with at least 2.6 nights per week of interrupted sleep).
Comment: These results don't yet prove that there is a cause-and-effect relation between light exposure at night and breast cancer. But, if such a link is confirmed, the magnitude of the risk would approach that of several other known risk factors for breast cancer.
— AS Brett
Published in Journal Watch General Medicine November 6, 2001
Citation(s):
Schernhammer ES et al. Rotating night shifts and risk of breast cancer in women participating in the Nurses' Health Study. J Natl Cancer Inst 2001 Oct 17 93 1563-1568.
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- Medline abstract (Free)
Davis S et al. Night shift work, light at night, and risk of breast cancer. J Natl Cancer Inst 2001 Oct 17 93 1557-1562.
- Original article (Subscription may be required)
- Medline abstract (Free)
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