Short answer: Melatonin has a half-life of about 45 minutes. Levels peak about 50 minutes after a dose. People vary, so treat these as estimates.
Melatonin is a hormone the brain makes in response to darkness. It helps set the timing of your 24-hour body clock and of sleep. As a supplement it is used for jet lag, for a sleep schedule that runs late (delayed sleep-wake phase disorder) and for some sleep problems.
Melatonin has no intake value or upper limit. In a systematic review of melatonin pharmacokinetics (Harpsøe 2015), studies used doses from 0.3 mg to 100 mg. These are study protocols, not a recommendation.
It is sold in the US as a dietary supplement. Under the 1994 dietary supplement law (DSHEA), FDA does not approve dietary supplements before they are marketed, and the maker, not FDA, is responsible for safety and labeling. The NIH National Center for Complementary and Integrative Health (NCCIH) notes that in several other countries melatonin is a prescription drug.
NCCIH says research suggests melatonin may help with jet lag and delayed sleep-wake phase disorder, but sleep guidelines found not enough strong evidence to recommend it for chronic insomnia. Short-term use appears safe for most people; long-term safety is unclear. Reported side effects include headache, dizziness, nausea and sleepiness. People with epilepsy or on blood thinners should use it only under medical supervision, and in older people it may stay active longer and cause daytime drowsiness. Labels are often wrong: in a 2023 study, 22 of 25 melatonin gummies were mislabeled: one had no detectable melatonin, and the rest held 74% to 347% of the stated amount.
A systematic review of 22 studies (Harpsøe 2015) concluded that melatonin peaks about 50 minutes after an immediate-release dose by mouth and has a half-life of about 45 minutes, by mouth or by vein. Individual studies ranged from 15 to 210 minutes to peak and 28 to 126 minutes for the half-life. Only about 15% of an oral dose reaches the blood, and age, caffeine, smoking, birth control pills and the drug fluvoxamine changed its pharmacokinetics.
Numbers shown are taken from the source above. Half-life varies from person to person.
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