Melatonin stabilises sleep onset time in a blind man without entrainment of cortisol or temperature rhythms.

The pineal hormone melatonin (N-acetyl-5-methoxytryptamine) is normally secreted at night: in animals it serves to transmit information about light-dark cycles to body physiology Suitable timed administration will alleviate 'jet-lag' severity ratings in humans, the major effect being to improve sleep. It has been suggested that this may be mediated by melatonin re-entraining the endogenous circadian oscillator. We have examined this possibility by feeding melatonin to a blind individual (HK) with a free-running temperature rhythm and a pronounced 35-day cycle in his ability to fall asleep at 'normal' times. Our results show a clear stabilizing effect of melatonin on sleep onset time with elimination of day time sleep, but no entrainment of rectal temperature or urinary cortisol rhythms. Thus melatonin may act on the timing mechanism of sleep onset, rather than as a entrainer of all circadian rhythms. It may well help shiftworkers to sleep at inappropriate phases of their circadian oscillators.