6Sulfatoxymelatonin

6-sulfatoxymelatonin, commonly abbreviated as aMT6s, is the major urinary metabolite of melatonin in humans. It forms when melatonin is hydroxylated at the 6 position and subsequently sulfated, a metabolic pathway predominantly occurring in the liver, with the sulfated product excreted in urine. Melatonin itself is produced by the pineal gland in a circadian rhythm, peaking at night.

Because urinary aMT6s mirrors nocturnal melatonin production, it is widely used as a noninvasive biomarker of endogenous melatonin output and circadian phase. Researchers collect overnight urine or a first-morning sample to estimate the total melatonin secretion; the pattern of excretion can relate to circadian markers such as dim-light melatonin onset (DLMO).

Measurement is typically done by immunoassays or liquid chromatography–mass spectrometry. Individual factors such as age, sex, health status, medications, and environmental light exposure can influence aMT6s levels, which should be considered when interpreting results.