Sleep as an Adaptation for Energy Conservation Functionally Related to Hibernation and Shallow Torpor

It is proposed that sleep constitutes another, albeit less well recognized, variation on the theme of mammalian dormancy—that is functionally related to hibernation and torpor, and it is suggested that they are functionally homologous processes.

Abstract

A vast amount of research on sleep has been generated during the past two decades and much has been learned about its phenomenology and underlying physiological and biochemical mechanisms. Yet the biological function of sleep remains enigmatic. A recent approach that might shed light on this question is to study sleep in relationship to behaviorally similar states—states whose biological function is obvious. Hibernation and shallow torpor are two such states. Although the mechanism underlying hibernation and torpor are not completely understood, there can be little doubt that their biological function is that of energy conservation. We propose that sleep constitutes another, albeit less well recognized, variation on the theme of mammalian dormancy—that is functionally related to hibernation and torpor. This chapter reviews the behavioral and physiological characteristics of sleep, shallow torpor, and hibernation in mammals and birds and proposes that they are functionally homologous processes. In addition, the chapter describes phylogenetic and ontogenetic correlations between sleep and endothermy, and sleep-dependent thermoregulatory adjustments associated with decreased metabolism consistent with an energy conserving role for sleep.