Background: In 2007, the International Agency for Research on Cancer classified shiftwork involving circadian disruption as being probably carcinogenic to humans. Research into the relationship between shiftwork and breast cancer is complicated because shiftwork may comprise several aspects: light at night; phase shift; sleep disruption; lifestyle factors (diet, physical activity and alcohol intake); and low vitamin D. Aim: To determine whether breast cancer is associated with one or more aspects of shiftwork. Methods: A population-based case-control study was conducted between 2009 to 2011; 1205 breast cancer cases were recruited through the Western Australian Cancer Registry and 1789 age-matched controls from the electoral roll. For every job held for at least 6 months, women completed a questionnaire on main tasks, duration, hours/ week and weeks/ year worked, and whether the job involved night or shift work, or work at unusual hours. Women were interviewed to obtain details about the shiftwork using OcclDEAS. We stratified analyses for morningness/eveningness, flexibility/rigidity and languidness/vigorousness of circadian rhythm, and menopausal status. Results: 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=004). For the other shiftwork factors, risks were marginally elevated and not statistically significant. Stratification by morningness/eveningness showed a tendency for the associations between breast cancer risk and light at night, phase shift, and sleep disruption to be higher among both neutral and morning preference women than among evening preference women. However, no interaction terms were statistically significant. No differences were observed by menopausal status. Conclusions: Phase shift (resulting from working multiple night shifts in a row) is possibly associated with breast cancer but the effect seen was not strong and needs replicating.
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