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Key Words sleep homeostasis, suprachiasmatic nucleus
Abstract Sleep and circadian rhythms are the primary determinants of arousal state, and torpor is the most extreme state change that occurs in mammals. The view that torpor is an evolutionary extension of sleep is supported by electrophy siological studies. However, comparisons of factors that influence the expression of sleep and torpor uncover significant differences. Deep sleep immediately following torpor suggests that torpor is functionally a period of sleep deprivation. Recent studies that employ post-torpor sleep deprivation, however, show that the post-torpor intense sleep is not homeostatically regulated, but might be a reflection of synaptic loss and replacement. The circadian system regulates sleep expression in euthermic mammals in such a way that would appear to preclude multiday bouts of torpor. Indeed, the circadian system is robust in animals that show shallow torpor, but its activity in hibernators is at least damped if not absent. There is good evidence from some species, however, that the circadian system plays important roles in the timing of bouts of torpor.
INTRODUCTION
Mammals that enter torpor undergo enormous changes in many physiological and neurophysiological variables. Deep torpor, or hibernation, involves a drop in regulated body temperature close to or even below the freezing point of water. Whole-body metabolism during deep torpor may be only 1 or 2% of basal values. Blood flow in most vascular beds falls very close to zero. Most of the nervous system becomes metabolically quiescent and electrically silent. Transcription and translation are virtually suspended. Yet, this state, which is as close to death as a living mammal can get, is spontaneously reversible. Over the winter phase of its annual cycle, a hibernator will enter repeated bouts of deep torpor that may last from an hour to weeks. Torpor bouts are punctuated by returns to normothermia that usually last a day or less. In addition to hibernators, there are many species that enter shallow torpor, also called daily torpor because it generally occurs during the inactive phase of the daily cycle. The physiological changes that occur during shallow torpor are less extreme in comparison with those in hibernation, but still dramatic in comparison with the homeostatic conditions of most mammals. Both daily torpor and hibernation raise many intriguing questions. This...