In short
Melatonin is a hormone produced primarily by the pineal gland in response to darkness, which signals circadian timing to the rest of the body and is sold over the counter in many countries as a sleep aid.
Endogenous melatonin secretion rises in the evening as light exposure drops, signaling the body's internal clock that it is nighttime, and falls after light exposure in the morning — a rhythm that can be disrupted by evening screen or bright-light exposure, shift work, and travel across time zones. Exogenous melatonin supplementation is best evidenced for shifting circadian timing (helping with jet lag and delayed sleep phase disorder) rather than as a general sedative, and clinical trial data supports doses considerably lower than what is commonly sold — effective circadian-shifting doses in research are often in the 0.5 mg range, while many commercial products contain 3, 5, or 10 mg.
Beyond sleep, melatonin also functions as a direct antioxidant and has receptors throughout the body, including in immune cells, which has generated separate research interest in roles beyond sleep regulation, including some preclinical work on oxidative stress and mitochondrial protection. This broader mechanistic research is real but considerably earlier-stage than melatonin's well-established sleep and circadian evidence base.
Marketing melatonin as a general longevity or anti-aging supplement, based on this broader antioxidant research, extends well beyond melatonin's actual approved and well-evidenced use as a circadian-rhythm and short-term sleep aid — there is no human trial showing that melatonin supplementation, at the doses commonly sold, extends lifespan or reverses a hallmark of aging in humans. It's also worth noting melatonin supplement content has been found in independent lab testing to sometimes differ substantially from the labeled dose, since regulation of supplement manufacturing consistency varies by market.