a hormone, CHNO, produced by the pineal body, that lightens skin pigmentation, inhibits estrus, etc.: its secretion is inhibited by sunlight
mela(no)- + ton(ic) + -in
See melatonin in American Heritage Dictionary 4
A hormone derived from serotonin and produced by the pineal gland that stimulates color change in the epidermis of amphibians and reptiles and plays a role in sleep, aging, and reproduction in mammals.