Sleep and Mammalian Hibernation: Homologous Adaptations and Homologous Processes?

Evidence from electroencephalographic, thermoregulatory and cellular neurophysiological studies suggests that sleep and hibernation may be homologous adaptations for energy conservation. However, despite the similarities between non-rapid eye movement (NREM) sleep and hibernation, the restorative function normally associated with slow wave sleep appears not to occur during hibernation, perhaps because of the low body temperature (Tb). Cellular neurophysiological studies also suggest that a bout of hibernation is not exclusively NREM sleep but is punctuated by periods of wakefulness. The entrance to hibernation involves both an inhibition of cortical activity and activation of hypothalamic regions, whereas the arousal from hibernation is primarily a hypothalamic function. Multiple neurochemical systems are affected by the arousal state change that occurs in hibernation, and a serotonergic-opiatergic interaction, in particular, may be important in regulating these events. Among regulated physiological systems affected by arousal state changes, the episodic respiration evident in hibernation shows striking similarities to the apneas observed during sleep in both humans and other mammals. Although the slight down-regulation of Tb and metabolism that accompanies the transition from wakefulness to NREM sleep may have served as a preadaptation for the evolution of hibernation among the mammals, increasing consideration must be given to the possibility that hibernation represents an arousal state distinct from any known normothermic arousal state.