Sleep is a behavioral and physiological state of rest that animals have acquired through their long evolutionary history to efficiently and successfully adapt to changes in the internal biological and external terrestrial environments. Until the middle of the 20th century, mammalian sleep was regarded as neural and physical hypofunction resulting from exhaustion, while neuroscientific progress since the discovery of REM sleep has revealed that it is an actively generated condition consisting of REM and non-REM periods, which are controlled through statespecific sleep centers and networks in the brain. In the present article, the author reviews current advances in physiological aspects of human sleep and describes three major components that cooperatively control sleep amount, timing, and occurrence, with a focus on sleep homeostatic, circadian, and wake-maintenance systems.
