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Melatonin is a hormone that is produced primarily by the pineal gland and secreted almost exclusively at night when it is dark.

Growing evidence also demonstrates that visible light, including artificial light, can acutely suppress melatonin production – a phenomenon often referred to as “circadian disruption” and commonly observed in shift workers. These observations led to the formulation of the ‘melatonin hypothesis’ suggesting that diminished secretion of melatonin might promote the development of cancer.

Triggered by this hypothesis, researchers accelerated their efforts to clarify whether increases in exposure to light at night could indeed increase cancer rates. Since then, epidemiologic data has continued to indicate that shift work is related to a modest increase in the risk of breast cancer and identified links to other cancers.

The growing evidence led WHO to re-classify night work as a probable carcinogen in 2019. In addition to cancer, several other chronic diseases have subsequently been linked by large prospective cohort studies to night work exposure (e.g., diabetes, cardiovascular disease, hypertension).

In this presentation, Schernhammer will review this evidence, along with a broad summary of epidemiologic studies of circadian disruption/sleep and chronic disease risk.

Light at Night and Human Health

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