melatonin -
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.
melatonin -
Any material similar in its chemistry and effect to the natural hormone.
OmegaWiki Dictionary
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melatonin A hormone of formula C13H16N2O2 that regulates the sleep-wake cycles in humans.