Energy conservation and sleep.

Phylogenetic and ontogenetic associations between sleep and endothermy are consistent with the hypothesis that sleep evolved in conjunction with endothermy to offset the high energetic cost of endothermy. The electrophysiological and thermoregulatory continuum of slow wave sleep, circadian torpor and hibernation substantiates a primordial link between sleep and energy conservation. Sleep constitutes a circadian and circannual rhythm of hypometabolic adaptation to biospheric energy cycles that is usually entrained through light-mediated suppression of melatonin secretion. When energy stores decline, energy is conserved by lowering Tb proportionally during sleep or by increasing the daily duration of sleep.