Melatonin: marvel or marker?

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Abstract
Melatonin blanches the skin of frogs, whitens the fur of hamsters, and sometimes makes the gonads atrophy. It is remarkable that such a hormone would be put forward as a defense against ageing. We have been examining excretion of the urinary metabolite of melatonin, 6-sulphatoxymelatonin (6-SMT), in 150 postmenopausal women, in 72 volunteers over the age of 60 years who complained of insomnia or depression, and in 20 healthy younger adult controls, aged 18–40 years. The acrophase or fitted peak of 6-SMT excretion was computed as a marker of the timing of the circadian system. Total daily excretion of 6-SMT was not significantly related to total sleep time, wake-within-sleep or sleep complaints. Nevertheless, whereas the 20 controls displayed a normal range of 6-SMT acrophases from 01.32 to 05.44 h, 42% of the postmenopausal women and 48% of the symptomatic elders had acrophases outside this normal range. Those volunteers with more deviant acrophases displayed more disturbed sleep and more sleep complaints. These data suggest that melatonin is a useful marker of circadian rhythm phase disorders, but suggest a need for more caution in melatonin administration.