Bioenergetic functions of sleep and activity rhythms and their possible relevance to aging.

The hypothesis is proposed that sleep constitutes a period of dormancy in which energy is conserved to partially offset the increased energy demands of homeothermy. Phylogenetic data indicate that the complete psysiological and behavioral manifestations of sleep are unique to homeotherms; furthermore "ontogeny recapitulates phylogeny" in the parallel development of slow wave sleep and thermoregulation as exemplified in the opossum. Thus, sleep constitutes a state of reduced metabolism that may represent a variation on the theme of dormancy, functionally lying on a continuum of energy conservation processes, ranging from inactivity and estivation to torpor and hibernation. The high amounts of sleep in infancy may involve conservation of energy and its consequent availability for growth. Decreased amounts of stage 4 and total sleep with aging in humans may represent reduced energy demands reflected by parallel declines in basal metabolic rate and physical activity. Disruptions of circadian rhythms of sleep and wakefulness in humans produce impairments in mood and performance independent of total amounts of sleep obtained, and reduce the amplitude of physiological rhythms. It is suggested that aging processes might also be affected by such disruptions in activity rhythms.