Circadian clues to sleep onset mechanisms

Thermoregulatory processes have long been implicated in initiation of human sleep. A meta-analysis of studies carried out under the controlled conditi ons of a constant routine protocol followed by nocturnal sleep revealed tha t heat loss indirectly measured by the distal-proximal skin temperature gra dient, was the best predictor variable for sleep onset latency (compared wi th core body temperature or its rate of change, heart rate, melatonin onset , and subjective sleepiness ratings). The cognitive signal of "lights out" induced relaxation, with a consequent shift in heat redistribution from the core to the periphery (as measured by an abrupt increase in skin temperatu re and a rapid fall in heart rate). These thermoregulatory changes took pla ce before sleep onset: sleep itself had minor further effects. Thus, when t he confounding long-lasting masking effects of lying down are controlled fo r, circadian thermoregulation initiates sleep, but does not appear to play a major role in its maintenance. (C) 2001 American College of Neuropsychoph armacology. published by Elsevier Science Inc.