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 conditions of a constant routine protocol followed by nocturnal sleep revealed that heat loss, indirectly measured by the distal-proximal skin temperature gradient, was the best predictor variable for sleep onset latency (compared with 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 temperatures and a rapid fall in heart rate). These thermoregulatory changes took place before sleep onset: sleep itself had minor further effects. Thus, when the confounding, long-lasting masking effects of lying down are controlled for, circadian thermoregulation initiates sleep, but does not appear to play a major role in its maintenance.