Rotating Night-Shift Work and the Risk of Breast Cancer in the Nurses' Health Studies

A total of 9,541 incident invasive breast malignancies were identified over 24 years of follow-up in two large cohort studies.

• Women with 30 years or more of shift work did not show a higher risk of breast cancer in the Nurses' Health Study.
• Among younger participants in the Nurses' Health Study II, 20 years or more of shift work was associated with a significantly higher risk of breast cancer.
• The hazard ratio for breast cancer risk was 2.15 for women with 20 years or more of shift work at baseline.
• Cumulative shift work of 20 years or more also showed a marginally significant association with breast cancer risk.
• Findings suggest that long-term rotating night-shift work could increase breast cancer risk, particularly when exposure occurs in young adulthood.

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