ASPO Abstracts
Shift work and working at night in relation to breast cancer incidence
Category: Molecular Epidemiology & Environment
Conference Year: 2020
Abstract Body:
Purpose of the study: Several studies have assessed night shift work in relation to breast cancer risk, with
inconsistent results; case-control studies have demonstrated a positive association while cohort studies have
been largely null. We aimed to assess the association between shift work that disrupts circadian rhythm, which
has been classified by the International Agency for Research on Cancer (IARC) as a probable carcinogen in
humans, and the risk of breast cancer in a large prospective cohort.
Methods: In the prospective Sister Study cohort, current and past occupational history was collected at baseline
(2003-2009) for 48,451 participants. We used Cox proportional hazards models to estimate adjusted hazard ratios
(HRs) and 95% confidence intervals (CIs) for the association between work schedule characteristics (including
duration of jobs with rotating shifts with or without nights, any work at night, or working irregular hours) and
incident breast cancer.
Results: During follow-up (mean=9.1 years), 3,191 incident breast cancer 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 shift: HR=1.08, 95% CI: 0.92-1.27; ever working at night: HR=1.01,
95% CI: 0.94-1.10; ever working irregular hours: HR=0.98, 95% CI: 0.91-1.06). While 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.
Keywords: Shift work, night shift, breast cancer, risk