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.…