The pineal gland secretes melatonin, a hormone linked to the sleep–wake cycle.
The pineal gland is a small endocrine organ near the centre of the brain, between the two halves of the thalamus. It converts serotonin into melatonin, with production normally increasing in darkness. Melatonin helps provide the body with a timing signal related to the daily sleep–wake rhythm.
The gland does not act as an independent master clock. Light information detected by the eyes reaches the suprachiasmatic nucleus of the hypothalamus, which helps regulate signals controlling pineal melatonin production. Evening light, especially blue-rich light, can suppress or delay melatonin release.
Historical descriptions sometimes gave the pineal gland a philosophical role, but modern physiology identifies it as a neuroendocrine structure. Melatonin is associated with circadian timing; it is not simply a general anaesthetic and does not guarantee sleep in every situation.