Shift Work and Working at Night in Relation to Breast Cancer Incidence.

Background: Night shift work has been classified by the International Agency for Research on Cancer as a probable carcinogen in humans. Several studies have assessed night shift work in relation to breast cancer risk, with inconsistent results.

Methods: In the prospective Sister Study cohort, current and past occupational history was collected for 48,451 participants. We used Cox proportional hazards models to estimate adjusted HRs and 95% confidence intervals (CI) for the association between baseline work schedule characteristics and incident breast cancer.

Results: During follow-up (mean = 9.1 years), 3,191 incident cases were diagnosed. We observed little to no increase in risk associated with work schedule characteristics (ever working rotating shifts: HR = 1.04, 95% CI, 0.91-1.20; ever working rotating night shifts: HR = 1.08, 95% CI, 0.92-1.27; ever working at night: HR = 1.01, 95% CI, 0.94-1.10; and ever working irregular hours: HR = 0.98, 95% CI, 0.91-1.06). Although short-term night work (>0 to 5 years vs. never: HR = 1.12; 95% CI, 1.00-1.26) and rotating shift work at night (>0 to 5 years vs. never: HR = 1.30; 95% CI, 1.05-1.61) were associated with increased breast cancer risk, working nights for more than 5 years was not associated with risk.

Conclusions: Overall, we observed little evidence that rotating shift work or work at night was associated with a higher risk of breast cancer, except possibly among those who participated in such work for short durations of time.

Impact: This study indicates that if night shift work is associated with breast cancer, the increase in risk is small.