Reviews critically the epidemiological literature that has addressed the possible relation between shift work and risk of breast cancer. Four published studies were identified that have directly investigated this relation, two cohort studies and two case-control studies. Each has different methodological strengths, and each has found some significant associations, sometimes with dose or duration response effects, albeit with varying size of risk and to different aspects of shift work. A potential mechanism for a relation between shift work and breast cancer risk would be via an effect of altered light exposure at night on levels of melatonin or other hormones that might affect cancer risk; this mechanism has not been established, however. Overall, the evidence for an association of breast cancer risk with shift work is appreciable but not definitive, and it remains unclear whether any association is causal or a consequence of confounding.
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