Night-shift work, breast cancer incidence, and all-cause mortality: an updated meta-analysis of prospective cohort studies.
Level 3 - non-randomized controlled study
Systematic review and meta-analysis of prospective cohort studies
PubMed 34775538 · doi:10.1007/s11325-021-02523-9
What was done
This was an updated systematic review and meta-analysis of 31 prospective cohort studies evaluating the association between night-shift work exposure, breast cancer incidence, and mortality outcomes. The analysis included 9.3 million participants, comprising 31,244 incident breast cancer cases, 12,728 cancer-related deaths, 7,882 cardiovascular deaths, and 30,807 all-cause deaths. Relative risks (RRs) were calculated for overall night-shift exposure, duration of exposure (>10 years), and shift type (rotating).
What was found
Overall, night-shift work exposure was associated with a modest increase in incident breast cancer in females (RR = 1.029, 95% CI 1.003–1.055). The risk was higher among workers with >10 years of exposure (RR = 1.086, 95% CI 1.032–1.142) and those working rotating night shifts (RR = 1.053, 95% CI 1.018–1.090). Night-shift work was also associated with increased cardiovascular mortality (RR = 1.031, 95% CI 1.006–1.057). Numerical effect sizes for all-cause mortality and total cancer-related mortality were not reported in the abstract.
Why it matters
This review provides pooled prospective evidence linking night-shift work—especially long-term or rotating schedules—to small increases in female breast cancer risk and cardiovascular mortality.
Limits
All included studies were observational cohorts, which are subject to residual confounding from lifestyle, reproductive, and socioeconomic factors. The relative risk increases are very small (2.9% to 8.6%), meaning minor unmeasured confounding could potentially explain the associations. Numerical results for all-cause and total cancer mortality were omitted from the abstract, and variations in how night shifts were defined across primary studies were not detailed.
Cited by
- supports Shift work is associated with adverse health outcomes including gastrointestinal dysfunction, increased cancer risk, and reduced longevity.