Does night work increase the risk of breast cancer? A systematic review and meta-analysis of epidemiological studies

Objective: To conduct a systematic review, with meta-analysis, of studies assessing the association between night work and the risk of breast cancer, using available epidemiological evidence. Method: Relevant studies were identified by searching several databases and the reference lists of retrieved articles. We combined the relative risks (RR) from individual studies using a random-effects model. Subgroup analysis was carried out as the data showed statistically significant heterogeneity. Results: Thirteen studies consisting of eight case-control studies and five cohort studies were included in the analysis. In the combined analysis of all studies, night work was associated with an increased risk for breast cancer (RR = 1.20, 95% CI = 1.08-1.33). The higher-quality studies showed a similar finding with a pooled RR of 1.40 (95% CI = 1.13-1.73). Both case-control studies (RR = 1.32, 95% CI = 1.17-1.50) and cohort studies (RR = 1.08, 95% CI = 0.97-1.21) showed a positive association between night work and the risk of breast cancer. No publication bias was found either from Begg's funnel plot (P = 0.086) or the Egger's test (P = 0.107). Additional well-conducted and large-scale epidemiological studies are needed. (C) 2013 Elsevier Ltd. All rights reserved.