The association between different night shiftwork factors and breast cancer: a case-control study

Background: Research on the possible association between shiftwork and breast cancer is complicated because there are many different shiftwork factors, which might be involved including: light at night, phase shift, sleep disruption and changes in lifestyle factors while on shiftwork (diet, physical activity, alcohol intake and low sun exposure). Methods: We conducted a population-based case-control study in Western Australia from 2009 to 2011 with 1205 incident breast cancer cases and 1789 frequency age-matched controls. A self-administered questionnaire was used to collect demographic, reproductive, and lifestyle factors and lifetime occupational history and a telephone interview was used to obtain further details about the shiftwork factors listed above. Results: A small increase in risk was suggested for those ever doing the graveyard shift (work between midnight and 0500 hours) and breast cancer (odds ratio (OR) 1.16, 95% confidence interval (CI) = 0.97-1.39). For phase shift, we found a 22% increase in breast cancer risk (OR 1.22, 95% CI = 1.01-1.47) with a statistically significant dose-response relationship (P = 0.04). For the other shiftwork factors, risks were marginally elevated and not statistically significant. Conclusion: We found some evidence that some of the factors involved in shiftwork may be associated with breast cancer but the ORs were low and there were inconsistencies in duration and dose-response relationships.