Night Work and Breast Cancer Risk Among Norwegian Nurses: Assessment by Different Exposure Metrics

A significant increase in breast cancer risk (odds ratio = 1.8) was observed in nurses working ≥5 years with ≥6 consecutive night shifts.

• No increased risk was found for nurses working ≥3 night shifts per month over long durations.
• Small, nonsignificantly increased risks were noted for exposure to night work for ≥12 years and a lifetime average of ≥4 night shifts per month.
• Nurses with ≥5 years of experience working ≥4 or ≥5 consecutive night shifts showed nonsignificantly increased risks of breast cancer.
• The results indicate a potential association between breast cancer risk and the number of consecutive night shifts worked.

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