The aim of this review is to evaluate the impact of sleep on breast cancer risk. Given the supposed protective role played by melatonin in breast cancer, it is interesting to study the effect of sleep, which is the moment of melatonin synthesis. Articles were extracted from the PUBMED database between 2000 and 2012 with the following keywords "sleep duration", "sleep quality", "breast cancer risk" and "melatonin". In total, 10 articles were selected. Most prospective cohort studies found a decrease in the risk of breast cancer varying from 38 to 72% for "long sleepers". Furthermore, a meta-analysis of the studies assessing the link between breast cancer risk and urinary concentration of 6-sulfatoxy-melatonin (6MT), which is melatonin's main metabolite, found a 34% decrease for patients with the highest 6MT concentration. Even though other studies are necessary to confirm these results, it seems already adequate to detect sleep disorders and to try to treat them.
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