A hormone produced by the pineal gland that regulates the sleep-wake cycle, increasing in darkness to promote sleepiness.
From Greek 'melas' (dark, black) + 'tonos' (tension) + '-in' (chemical suffix). Named because it lightens skin in frogs.
Melatonin is your brain's sleep signal — when it gets dark, the pineal gland releases it to tell your body 'time for bed!'
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