The Function of Sleep

Abstract

Many hypotheses have been put forward for the functions of sleep, but none have been either proved or falsified. Rest and active states are ubiquitous in animals. However, it is impossible to say whether rest states in invertebrates and in ectothermic vertebrates are homologous with nonrapid eye movement (NREM) and rapid eye movement (REM) sleep in endotherms. Once functional correlates of sleep states in endotherms are established, it will be possible to investigate whether those same functions are associated with the rest states in other species. With respect to NREM and REM sleep, there is an assumption that they serve a restorative function. But, what is being restored is not clear. Cognitive abilities degrade with even mild sleep loss; but why it is so is not known. Two major lines of thinking about sleep function are that they are involved in the restoration of brain energy metabolism or that they are involved in processes of synapse and circuit maintenance including the consolidation of memories. These functions are not mutually exclusive, and evidence in support of them is discussed. A critical question that must be asked when considering any putative function of sleep is – Why must the brain be taken off-line to accomplish that function?