The functions of sleep

Abstract

The function or functions of sleep are not known, but it is likely they are principally for the brain. Physiology of organs apart from the nervous system is very similar in quiet wake and sleep, but the activity of the brain is very different between sleep and wake and extremely different between the two sleep states. Critically the brain must be taken off-line to accomplish sleep functions putting the individual at great risk. Several hypotheses of sleep function are being investigated and are discussed in this review. They include restoration of cerebral energy reserves, neuroplasticity and synaptic downscaling, memory consolidation, and glymphatic clearance of metabolic wastes. Rest states are virtually ubiquitous among animals, but we don't know if they or any of their functions are homologous with mammalian and avian sleep. Evolution may have added species specific functions to rest or sleep states, for example hibernation as an extension of ubiquitous mammalian sleep. So, there may be primitive/common functions of rest/sleep and derived functions. A critical question to ask of any hypothetical sleep function is why the brain has to be taken off-line to accomplish that function. Putative sleep functions described by different hypotheses may be synergistic and linked. The functions of NREM and REM sleep are most likely different, but could be related and interdependent in a homeostatic relationship.