Objective This study aimed to (i) investigate associations between different characteristics of exposure to shift work and breast cancer risk and (ii) identify effect modifiers.

Methods Comprising 42 379 women from the Finnish Public Sector cohort, this longitudinal study included data from survey responses, payroll-based working hour records, and national breast cancer registry data. Cause-specific Cox proportional hazards models were used to estimate hazard ratios (HR), adjusting for age, marital status, socioeconomic status, living with a child, lifestyle, and occupational factors. Stratified analyses and interaction tests were performed to assess effect modification.

Results Over an average follow-up of 9.8 years, 945 women were diagnosed with breast cancer. Among women aged ≥50 years, increased risk was associated with working >3 consecutive night shifts [hazard ratio (HR) 1.64, 95% confidence interval (CI) 1.22–2.21] and evening shifts (HR 1.28, 95% CI 1.03–1.59). Sleep problems and fatigue at work modified the association between night shift work and breast cancer risk, whereas psychological distress and diurnal preference did not. Permanent night shift workers reporting sleep problems (HR 1.76, 95% CI 1.01–3.09) or fatigue at work (HR 1.90, 95% CI 1.08–3.35) had a higher risk compared to day workers with similar symptoms.

Conclusions Women aged ≥50 years showed increased breast cancer risk for several shift work characteristics. Permanent night shift work was especially associated with increased risk among those reporting sleep problems or fatigue. These findings may help identify groups needing closer occupational health monitoring, though causal inference is limited by the observational design. Intervention studies are required to determine whether changes in work schedules or management of sleep problems and fatigue could reduce risk.

Key terms chronotype; cohort study; fatigue; Finland; night shit work; night work; payroll data; psychological distress; shift work; shift work schedule; sleep disturbance; survey data

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