Objectives Using two large prospective data sets, we explored associations between rotating nightshift work and breast cancer risk.

Method Among 193 396 women from the Nurses’ Health Study (NHS and NHS2) cohorts, we documented 5575 (NHS) and 2869 (NHS2) incident invasive breast cancer cases as well as 696 (NHS) breast cancer deaths over 22 years of follow-up. Compared to our initial analysis within NHS, which was based only on 10 years of follow-up and showed that 30+ years of rotating nightshift work was associated with a 36% significant increase in breast cancer risk, we added 12 years of follow-up, which were accrued for the most part after nurses’ retirement.

Results In these extended analyses, 30+ years of rotating night shift work was no longer associated with breast cancer risk (multivariable RR_30+yrs=0.95, 95% CI 0.77–1.17; p_trend=0.95), and only insignificantly associated with breast cancer mortality (multivariable HR_30+yrs=1.50, 95% CI, 0.95–2.36). By contrast, in the younger NHS2 cohort, baseline 20+ years of rotating nightshift work was associated with a significantly increased risk of breast cancer (RR_20+yrs =2.11, 95% CI 1.21–3.66; p_trend=0.22). Updated rotating night shift work exposure was also associated, albeit non-significantly, with a modest increase in risk of breast cancer (RR_20+yrs=1.33, 95% CI0.93–1.89; p_trend=0.68).

Conclusions Taken together, these results suggest that long-term rotating night shift work particularly early in career may be associated with an increased risk of breast cancer, which appears to diminish after nightshift work ceases. Future studies are needed to confirm these findings and should explore the potential for tailored risk factor counselling in nightshift workers.