SLEEP AND MAMMALIAN HIBERNATION - HOMOLOGOUS ADAPTATIONS AND HOMOLOGOUS PROCESSES

Evidence from electroencephalographic, thermoregulatory and cellular n europhysiological studies suggests that sleep and hibernation may be h omologous adaptations for energy conservation. However, despite the si milarities between non-rapid eve movement (NREM) sleep and hibernation , the restorative function normally associated with slow wave sleep ap pears not to occur during hibernation, perhaps because of the low body temperature (T(b)). Cellular neurophysiological studies also suggest that a bout of hibernation is not exclusively NREM sleep but is punctu ated by periods of wakefulness. The entrance to hibernation involves b oth an inhibition of cortical activity and activation of hypothalamic regions, whereas the arousal from hibernation is primarily a hypothala mic function. Multiple neurochemical systems are affected by the arous al state change that occurs in hibernation, and a serotonergic-opiater gic interaction, in particular, may be important in regulating these e vents. Among regulated physiological systems affected by arousal state changes, the episodic respiration evident in hibernation shows striki ng similarities to the apneas observed during sleep in both humans and other mammals. Although the slight down-regulation of T(b) and metabo lism that accompanies the transition from wakefulness to NREM sleep ma y have served as a preadaptation for the evolution of hibernation amon g the mammals, increasing consideration must be given to the possibili ty that hibernation represents an arousal state distinct from any know n normothermic arousal state.