ABSTRACT
Sleepiness can be viewed as a basic physiological need state like hunger or thirst that is vital to human survival. Deprivation of sleep causes sleepiness, and as eating and drinking satisfy hunger or thirst, sleeping reverses sleepiness. Under normal circumstances, severe sleep-deprived states do not occur as normal homeostatic and behavioral regulation (like eating and drinking reverse hunger and thirst) modulate conditions to facilitate sleeping before severe deprivation states develop. The neurological substrates of sleepiness and the specific nature of this physiological need state have yet to be ascertained.
