Circadian disrupting exposures and breast cancer risk: a meta-analysis.

PURPOSE Shift work, short sleep duration, employment as a flight attendant, and exposure to light at night, all potential causes of circadian disruption, have been inconsistently associated with breast cancer (BrCA) risk. The aim of this meta-analysis is to quantitatively evaluate the combined and independent effects of exposure to different sources of circadian disruption on BrCA risk in women. METHODS Relevant studies published through January 2014 were identified by searching the PubMed database. The pooled relative risks (RRs) and corresponding 95 % confidence intervals (CIs) were estimated using fixed- or random effects models as indicated by heterogeneity tests. Generalized least squares trend test was used to assess dose-response relationships. RESULTS A total of 28 studies, 15 on shift work, 7 on short sleep duration, 3 on flight attendants, and 6 on light at night were included in the analysis. The combined analysis suggested a significantly positive association between circadian disruption and BrCA risk (RR = 1.14; 95 % CI 1.08-1.21). Separate analyses showed that the RR for BrCA was 1.19 (95 % CI 1.08-1.32) for shift work, 1.120 (95 % CI 1.119-1.121) for exposure to light at night, 1.56 (95 % CI 1.10-2.21) for employment as a flight attendant, and 0.96 (95 % CI 0.86-1.06) for short sleep duration. A dose-response analysis showed that each 10-year increment of shift work was associated with 16 % higher risk of BrCA (95 % CI 1.06-1.27) based on selected case-control studies. No significant dose-response effects of exposure to light at night and sleep deficiency were found on BrCA risk. CONCLUSIONS Our meta-analysis demonstrates that circadian disruption is associated with an increased BrCA risk in women. This association varied by specific sources of circadian disrupting exposures, and a dose-response relationship remains uncertain. Therefore, future rigorous prospective studies are needed to confirm these relationships.