Melatonin is a hormone (N‑acetyl-5‑methoxytryptamine) produced especially at night in the pineal gland. Its secretion is stimulated by darkness and inhibited by light. Melatonin, an indole, is synthesized from tryptophan via serotonin. The suprachiasmatic nuclei (SCN) of the hypothalamus have melatonin receptors and melatonin may have a direct action on SCN to influence circadian (sleep) rhythms.