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Melatonin (5-methoxytryptamine) is an indoleamine produced primarily by the pineal gland, which is secreted exclusively during the dark phase of the light-dark cycle in humans. Several decades ago, reports indicated that melatonin possesses oncostatic properties, leading to novel hypotheses that diminished secretion of melatonin might promote the development of cancer. Growing evidence also demonstrates that visible light, including electric light, can acutely suppress melatonin production– a phenomenon often referred to as ‘circadian disruption’ particularly if it occurs at night, as commonly observed in shift workers.In 2007, the International Agency for Research on Cancer classified shift work as a possible carcinogen, based on convincing experimental evidence and supportive, but still limited, epidemiologic data. Indeed, experimental data has consistently demonstrated that circadian disruption can promote carcinogenesis in animals; specifically, exposure to light at night and phase shifts in the light-dark cycle have accelerated tumour development in rodents. In humans, epidemiologic data continues to accumulate, with the majority of existing studies indicating that shift work is related to a modest increase in the risk of breast cancer. Initial studies have identified links between shift work and other cancers as well, although this evidence is very limited.In this presentation, I will review epidemiologic studies of circadian disruption/sleep and cancer risk, including some preliminary studies on cancer survival, and with additional emphasis on circadian phase biomarkers (melatonin/cortisol).