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6-OH-Melatonin-Sulfate, also called 6-sulfatoxymelatonin or aMT6s, is the main breakdown product of melatonin in humans. More than 90 percent of the melatonin your body produces is eventually converted into this form and excreted in the urine. Because of that, measuring aMT6s in urine gives a reliable, noninvasive estimate of how much melatonin your body made over a given period of time.
Melatonin is a hormone produced by the pineal gland in the brain. Its release is tightly controlled by your internal clock, also called the circadian rhythm. The circadian system coordinates sleep, hormone release, body temperature, metabolism, and many other processes over a 24 hour cycle.
Light exposure in the evening suppresses melatonin. Darkness stimulates it. The normal pattern is low levels during the day, rising in the evening, peaking at night, and falling toward morning.
After melatonin is released into the bloodstream, it is processed primarily in the liver. First, phase I enzymes known as cytochrome P450 enzymes, especially CYP1A2, chemically modify melatonin through a reaction called hydroxylation. Hydroxylation means adding a small chemical group containing oxygen and hydrogen.
Next, phase II enzymes called sulfotransferases, mainly SULT1A1, attach a sulfate group to the molecule. This step, called sulfation, makes the compound more water soluble so it can be excreted in urine. The final product of these steps is 6-hydroxymelatonin sulfate.
Because aMT6s reflects the integrated output of melatonin over several hours, a single urine sample collected overnight or over 24 hours can substitute for repeated nighttime blood draws. This makes testing easier and more practical, especially in children, older adults, and long term circadian rhythm research.
It is also useful when the timing of the internal clock is uncertain, such as in shift work disorder, jet lag, or non 24 hour sleep wake rhythm disorder. In these conditions, the amplitude, meaning the strength of the rhythm, or the phase, meaning the timing of the rhythm, may be altered.
Low urinary aMT6s levels are commonly seen in older adults with insomnia. In some studies, individuals with very low nighttime excretion responded better to melatonin replacement therapy than those with higher baseline levels. Levels are very high in early childhood, stabilize in adolescence, and gradually decline after midlife. This age related decrease may partly explain why sleep becomes lighter and more fragmented over time.
Reduced aMT6s can also reflect impaired serotonin production. Serotonin is a neurotransmitter derived from the amino acid tryptophan and serves as the direct precursor to melatonin. If serotonin synthesis is impaired, melatonin production falls and urinary aMT6s drops accordingly. Biogenic amines are signaling molecules derived from amino acids, including serotonin and dopamine.
Autonomic nervous system dysfunction can also reduce levels. The autonomic nervous system controls involuntary functions such as heart rate and blood pressure and communicates with the pineal gland through sympathetic nerve fibers. In peripheral autonomic failure, melatonin secretion and aMT6s excretion can be markedly reduced.
Liver disease can alter aMT6s patterns because melatonin is metabolized in the liver. In cirrhosis, the timing of melatonin breakdown may be delayed, which can distort its normal rhythm. In this case, abnormal results may reflect both impaired liver clearance and circadian disruption.
Certain medications also matter. Beta 1 adrenergic blockers such as atenolol can blunt or abolish the normal 24 hour rhythm of aMT6s by interfering with sympathetic signaling to the pineal gland. Genetic variation in CYP1A2 or SULT1A1 may also influence how rapidly melatonin is metabolized.
High urinary aMT6s most commonly reflects melatonin supplement use. When melatonin is taken orally, it undergoes first pass metabolism in the liver before entering systemic circulation. First pass metabolism means the drug is extensively processed by the liver before it reaches the rest of the body. This can generate large amounts of urinary aMT6s without necessarily indicating abnormally high circulating melatonin levels.
Elevated levels can also be seen in young children, who naturally produce more melatonin. Because of this metabolism effect, urine aMT6s is not a reliable tool for fine tuning supplement dosing.
aMT6s offers a practical window into circadian biology. Circadian misalignment has been linked to cardiometabolic disease, cognitive decline, mood disorders, and impaired immune function. By assessing integrated melatonin production, this biomarker helps determine whether the biological signal of night is intact, delayed, blunted, or absent.
That information can guide targeted interventions such as optimizing light exposure, adjusting behavior, or in selected cases, using melatonin therapy.
Within person biological variation for 24 hour urinary aMT6s is modest, around 12 percent. This means repeat measurements can improve reliability. There are no consistent gender differences in reference intervals, although laboratory studies suggest sex differences in sulfation activity at the cellular level.