Sleep and hypometabolism

Metabolic rate (MR) of endotherms is lower during sleep than quiet wakefulness (W), largely as a result of a regulated lowering of body temperature (Tb) during slow-wave sleep (SWS). In mammals this decrease in regulated Tb is evidenced by a lowered hypothalamic thermosensitivity. In avian thermoregulatory systems spinal thermosensitivity is lower in SWS than in W. In rapid eye movement sleep (REMS) there is a severe inhibition of the thermoregulatory systems. Hypothalamic cells that are thermosensitive during W become less so during SWS and become totally insensitive to temperature during REMS. The adjustments in thermoregulatory systems of endotherms that result in lower Tb and MR during SWS have been shaped by natural selection in different species to produce a variety of adaptations for energy conservation. Bouts of torpor, hibernation, or nocturnal hypothermia consist mostly of SWS, and these states involve a downward resetting of hypothalamic thermosensitivity in mammals and spinal thermosensitivity in birds. A physiological mechanism underlying this change in regulation of Tb may be related to the regulation of breathing and acid–base balance. Retention of CO2 occurs at the onset of sleep, shallow torpor, and hibernation and release of excess CO2 occurs when these states are reversed by arousal. Increased plasma CO2 may have a direct effect on hypothalamic neurons involved in thermoregulation, resulting in a decline in regulated Tb.