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Melatonin is a chemical found in the human body and is part of the biological machinery that regulates the sleep-wake cycle of an individual over the course of a day. It is also found in other living species, including animals, plants, and microorganisms. In humans, high levels of melatonin are secreted by the pineal gland at night, which causes drowsiness. Medications containing melatonin have been used to treat jet lag and insomnia, although efficacy is still debatable.