Comparative aspects of energy metabolism, body temperature and sleep.

Comparative studies of energy metabolism, thermoregulation and sleep indicate that down-regulation of hypothalamic thermosensitivity and metabolic rate is initiated at sleep onset and extended as animals enter circadian torpor or shallow hibernation, which are characterised by predominant patterns of electrophysiological slow wave sleep (SWS). Body weight, hibernation and euthermic sleep propensity are all endogenous circannual rhythms. Circadian torpor is also an endogenous rhythm, during which body temperature (Tb) and metabolic rate (MR) in small-to-medium sized birds are regulated at levels proportional to preceding diurnal energy reserves. In large birds total sleep and SWS increase during fasting without change in Tb or MR. In humans SWS is also increased by fasting and raised brain temperature (as indexed by tympanic temperature). The increased propensity for SWS with elevation of Tb, the reduction in MR and Tb during SWS, and the thermoregulatory and electrophysiological continuities between SWS, circadian torpor and hibernation are consistent with energy conservation theories of SWS function, and with the hypothesis that the induction of sleep by moderate peripheral and central heating is an active homeothermic response preventing hyperthermia (Obal, 1984). Therefore, changes in thermal and energy balance should be taken into consideration in studies of sleep and exercise.