Melatonin

Melatonin is a hormone produced mainly by the pineal gland in response to darkness. It signals biological night and helps time circadian rhythms, including the sleep–wake cycle.

Pineal gland: An endocrine gland in the brain that secretes melatonin, especially during darkness. It is the principal source of circulating melatonin in humans.
Melatonin supplement: An oral preparation containing melatonin, sold or prescribed to influence sleep timing. Supplemental melatonin is used to shift circadian timing or support sleep onset.
Circadian clock: An internal biological timekeeping system that generates near-24-hour rhythms and synchronizes them with the environment. Melatonin is a rhythmic output and timing cue of this system.
Adenosine: A signaling molecule that accumulates during wakefulness and contributes to homeostatic sleep pressure. Adenosine tracks time awake, whereas melatonin mainly signals circadian night.
Suprachiasmatic nucleus: A hypothalamic brain region that coordinates circadian rhythms and receives direct input from the eyes. It relays circadian timing signals along the pathway controlling pineal melatonin secretion.
Jet lag: A temporary mismatch between internal circadian rhythms and local time after rapid travel across time zones. Timed melatonin can help realign the body clock after eastward or westward travel.
Light: Visible electromagnetic radiation that stimulates photoreceptors and provides a major environmental timing cue. Light exposure at night suppresses melatonin secretion and can shift circadian timing.
Cortisol: A steroid hormone involved in stress responses and metabolism, with a daily rhythm typically opposite melatonin's. Its usual morning rise contrasts with melatonin's nighttime rise.
Retinohypothalamic tract: A neural pathway carrying light information from retinal cells to the suprachiasmatic nucleus. It lets environmental light suppress melatonin production through the central clock.
Delayed sleep phase disorder: A circadian rhythm sleep disorder in which sleep onset and waking occur substantially later than desired. Carefully timed melatonin can advance sleep timing in some cases.

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