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usually uncountable, plural melatonins
A hormone, related to serotonin, that is secreted by the pineal gland, and stimulates colour change in the skin of reptiles, and is involved in the sleep/wake and reproductive cycles in mammals. quotations examples
How exactly melatonin relates to sleep is still not understood.
2019, Bill Bryson, The Body: A Guide for Occupants, Black Swan (2020), page 310
Any material similar in its chemistry and effect to the natural hormone. examples