Night shift work and breast cancer incidence among women: a population-based cohort study

BackgroundNight shift work may increase breast cancer incidence, but evidence is inconclusive. We investigated the effect of night shift work on breast cancer incidence, considering life course night shift work characteristics, sleep and menopausal status. MethodsWe analyzed two UK Biobank cohorts, using Cox proportional hazards regression models. The first cohort (20,860 participants) investigated the effect of night shift work on breast cancer incidence (688 cases) from baseline (2006-2010) to the end of follow-up (2022), stratified by sleep patterns and menopausal status. The second cohort (64,539 participants) investigated night shift work characteristics (duration, frequency, length, consecutive shifts, rest days) and breast cancer incidence (4,455 cases), using retrospective data from participants first job to the end of follow-up. ResultsNo effect of night shift work on breast cancer incidence was observed in the first cohort (hazard ratio (HR) 0.95, 95% CI: 0.85-1.06) or in the second cohort (HR 1.05% CI: 0.96-1.13). Stratification by sleep patterns and menopausal status showed no modifying effect. No night shift work characteristics were linked to breast cancer incidence. ConclusionsOur findings suggest no effect of night shift work on breast cancer incidence, even when considering sleep patterns, menopausal status, and detailed life course characteristics.