Melatonin helps regulate sleep-wake or circadian rhythms. Normally, production of melatonin by the pineal is stimulated by darkness and inhibited by light.

In recent times, melatonin has become available as a drug and a supplement. It appears to have some use against insomnia and jet lag.

To synthesize melatonin, serotonin is converted to N-acetyl serotonin by the enzyme 5-HT N-acetyltransferase. N-acetyl serotonin is then converted to melatonin by the enzyme 5-hydroxyindole-O-methyltransferase. See serotonin for details on how tryptophan is converted to serotonin.