Introduction Night shift work was re-reclassified in 2019 by IARC/WHO as a probable human carcinogen (Group 2A) for humans, with limited epidemiological evidence for breast, prostate and colorectal cancer.

Objectives The objective of this talk is to provide an overview of the evidence on night shift work and cancer in epidemiological studies with a focus on breast cancer, to discuss strengths and limitations of existing studies and summarize areas for future research studies and policy actions.

Methods Among others, results from a pooled analysis of 5 population-based case-control studies of breast cancer using a common definition of night work (at least 3 h between midnight and 5 a.m.) will be presented. Results from a systematic Cochrane review on the effect of years of night shift work on cancer incidence will be summarized.

Results Women who ever worked at night had higher odds for breast cancer compared to never night workers (OR 1.12 95% CI 1.00–1.25) in the pooled analysis. The risk was higher among pre-menopausal women (1.26; 1.06–1.51), high shift-work intensity and ER+ tumors. Our systematic review included 20 studies on breast cancer (12 case-control and 8 cohort studies). In preliminary meta-analysis, a non-linear dose-response relationship was found, with a 7% risk increase in breast cancer after 20 years of night work (95% CI: 1.01–1.15). This finding was stronger in studies that reported lifetime occupational history and case-control studies.

Conclusions Night shift work of high intensity and long duration tends to increase the risk of breast cancer. Findings are stronger in studies with lifetime occupational history, among pre-menopausal women and positive hormone receptor subtypes. Other shift work research domains that need to be considered in future studies include 1) patterns of night work schedules 2) susceptible groups e.g. chronotype 3) critical exposure windows 4) co-exposures with occupational carcinogens.