In 2007, the International Agency for Research on Cancer classified shift work that involved circadian disruption as probably being carcinogenic to human breast cancer (Group 2A), which bases on limited epidemiological evidence. Five relevant epidemiological studies have been published since 2007. However, a systematic and quantitative assessment of published studies is not available.

Three cohort and seven case-control studies were identified from the MEDLINE database from 1970 to August 2012 without restrictions and by reviewing reference lists from retrieved articles. Studies that reported relative risk estimates with 95% confidence intervals (CIs) for the associations of interest were included. Except the duration of night-shift, available data of night-shift frequency and cumulative night-shift were also extracted from these studies. Summary estimates of association were obtained using the fix or random-effects models.

The pooled relative risk (RR) per 5-year of night-shift was 1.05 [95% confidence interval (CI): 0.87–1.28] for night-shift work duration. Cumulative night-shift was positively associated with breast cancer risk (RR = 1.13 per 500 night-shift; 95% CI = 1.07–1.21; four studies), night-shift frequency was not associated with breast cancer risk (RR = 1.01 3-shift/month, 95% CI = 0.99–1.03; three studies).

Findings from this meta-analysis indicate that cumulative night-shift is positively associated with breast cancer risk, although the night-shift work duration didn’t associate with breast cancer risk. Night-shift frequency is not associated with breast cancer risk. Further studies are needed to confirm these findings.