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Hormone secreted by the pineal gland of most vertebrates. It appears to be important in regulating sleeping cycles; more is produced at night, and test subjects injected with it become sleepy. Melatonin may be involved in seasonal affective disorder. In mammals other than humans, melatonin may act as a cue to breeding and mating in season.
This entry comes from Encyclopædia Britannica Concise. For the full entry on melatonin, visit Britannica.com.