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Electrical light, which has been a widespread benefit only during about the past 100 years, is new in an evolutionary perspective, and has among priceless advantages given humans the ability to work during the normal dark part of the day (evening, night and early morning). In the late 1980s Stevens1 suggested that exposure to light at night may increase the risk of breast cancer. The initial hypothesis was that exposure to light at night suppresses pineal melatonin secretion, potentially reversing the normally oncostatic properties of the hormone and increasing circulating oestrogens associated with breast carcinogenesis. Later, this hypothesis was further applied to aspects of desynchronisation of the master circadian pacemaker (suprachiasmatic nucleus) with peripheral oscillators due to slow adaption and synchronisation to rapid transitions between different shift schedules, different expressions in circadian genes, and sleep deprivation followed by night shift work in initiating or promoting breast cancer and probably other cancers.2 The first epidemiological studies testing Steven's hypothesis were published in 2001, and all three independent studies from Denmark and the USA showed significantly increased breast cancer risks, including a dose-related increase in risk with increasing years of non-daytime work.3 In October 2007, an expert group from 10 countries convened by the International Agency …
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