Introduction Night work is increasingly common in the modern world. In 2007, the International Agency for Research on Cancer (IARC) classified shift work that involves circadian disruption as probably carcinogenic to humans based on limited evidence from eight epidemiologic studies on breast cancer, in addition to sufficient evidence from animal experiments. The overall evidence was further supported by increased risk in studies of flight attendants, who may be exposed to both shift-work and jet-lag, though night work was not the primary focus in these studies.

Methods Based on a critical review of the scientific literature it is the aim of to present the current epidemiologic evidence and to identify eventual threshold effects, e.g. by duration of night shifts, number of consecutive shifts and menopausal status.

Results After 2007, over 18 new studies with focus on night work and breast cancer are published, which more than triples the number of studies focusing on this issue since the IARC evaluation. The assessment of night shift work is different in all of the studies, which may attenuate the observed increased risk and hinders meta-analysis. There is some evidence that long duration and or high number of consecutive night shifts has impact on the magnitude of increased breast cancer risk. There is also some evidence that increased risk is primarily observed in women who have had nigh work in young adulthood, and in pre-menopausal women.

Conclusion Overall, there is a tendency of increased risk of breast cancer after long-term night shift or after shorter periods with many consecutive shifts. More studies using harmonised definitions of night work metrics is needed. Finally, evidence based preventive interventions are warranted.