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

Background Night 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.

We 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.

No 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.

Our findings suggest no effect of night shift work on breast cancer incidence, even when considering sleep patterns, menopausal status, and detailed life course characteristics. Competing Interest Statement The authors have declared no competing interest. Funding Statement The author(s) received no specific funding for this work. Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: The UK Biobank is an open access resource, available at https://www.ukbiobank.ac.uk/researchers/. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes Data availability The UK Biobank is an open access resource, available at https://www.ukbiobank.ac.uk/researchers/.