Melatonin is an indoleamine synthesized from tryptophan and secreted from the pineal gland during the night. Peak concentrations in blood reach 60–80 pg/mL at around 2–3 a.m. in humans, whereas daytime levels are barely detectable. The half-life of melatonin in the blood is ∼ 50–60 min. As a result of first-pass metabolism in the liver, 90 % of melatonin is catabolized to 6-sulfatoxymelatonin, which is excreted in the urine. Most of the melatonin (e.g., 70 %) circulating in the bloodstream is bound to albumin. The absolute oral bioavailability of melatonin (2–4 mg) is 15 % with peak levels (2–4 ng/mL) being reached within ∼ 1 h of ingestion. The bioavailability of lower amounts (i.e., micrograms) of oral melatonin varies widely. Melatonin is regularly consumed from commercially available nutritional supplements by millions of people throughout the world primarily for sleep problems and/or jet lag. The long-term health implications of this are unknown.