Low and high melatonin excretors among healthy individuals.

To meet the need of establishing firm normative data regarding the secretion/excretion of human melatonin, nighttime urinary melatonin of 16 healthy volunteers was measured in samples collected monthly over a period of 1 year. Low melatonin excretors (N = 8) were distinguished from high melatonin excretors (N = 8), based on a cut-off mean melatonin value of 0.25 nmol/l. There was no overlap in any of the monthly melatonin values between the two groups, while their annual rhythms of melatonin excretion were not different in shape. Since no obvious factors (age, sex, height, weight, etc.) were responsible for the observed differences, the distinction between low and high nocturnal excretion and by inference secretion of melatonin most likely reflects genetically determined variable levels of the noradrenergic secretory drive and/or variable N-acetyltransferase/hydroxyindole-O-methyltransferase enzymatic activity during the night.