Melatonin is initially metabolized in the liver to 6-hydroxy-melatonin (aMT6S, M6S), then conjugated to sulphate and excreted into urine.

The production and excretion of 6-sulfatoxy- melatonin follows an individual’s circadian rhythm, like melatonin. Measurement of 6-sulfatoxy- melatonin may be used as an index of pineal function.

For both, clinical and research purpose, the timing of nocturnal onset of 6-sulfatoxy-melatonin has been proposed as a reliable and robust marker of the circadian phase in order to diagnose disorders with an underlying circadian component e.g. Advanced or Delayed Sleep Phase Syndrome, winter depression, shift work adaptation etc. Moreover, the position of the circadian phase is imperative for the implementation of therapies whose efficacy varies with the phase of the underlying endogenous circadian pacemaker such as the administration of bright light and/or exogenous melatonin.

was discontinued