Introduction International Agency for Research on Cancer (IARC) has concluded that night shift work is probably carcinogenic to humans.

Objectives To examine the association of shift work with breast cancer among women in the Finnish public sector.

Methods We investigated the associations of night and shift work, and potential confounders, with incident breast cancer using a survey (N=64 082, mean age 42,3 years) and payroll cohort (N=17 286, 2008-) of the Finnish Public Sector study (FPS). Survey and payroll employees (baseline 2000–2012) were linked to National Cancer Register and followed-up for breast cancer to the end of 2016. Hazard ratios (HR) and confidence intervals (95% CI) from Cox proportional hazard regression models were calculated, including adjustment for age, SES, number of children, smoking, alcohol and body mass index.

Results 2057 incident cases of breast cancer were identified. Compared to day work, self-reported shift work without nights at baseline was associated with an increased risk of breast cancer during a follow-up period of less than 10 year, after adjustment (HR 1.33, 95% CI 1.15–1.55). In contrast, follow-up for > = 10 years showed no increased HR. Night shift work was most notably (HR 2.05, 95% 1.04–4.01) associated with breast cancer among employees 50 years or older after 10 years of follow-up. The risk of breast cancer tended to depend on earlier exposure time. The results of the more recent payroll sub-cohort showed no association of shift work and breast cancer, including intensity and duration of night shift work.

Conclusion This study gives some indications of an increased risk of breast cancer among subgroups of shift-working Finnish public sector employees. However, insufficient information on past exposure to, and intensity of night work, limits the ability to draw firm conclusions.