Sleep is a physiological condition essential to life, present in all living organisms with a neuronal and glial network. Its functions, not fully understood, include the conservation of energy, the regulation of our immune system and brain function through the modulation of synaptic plasticity and the elimination of substances accumulated during wakefulness. Sleep in human is characterized by two exclusive states, the non-REM and the REM -paradoxical- sleep, whose occurrence is driven by a cyclic organi zation. Sleep and wakefulness result from complex mechanisms involving a hetero geneous transition of different brain structures from one physiological state to another. The sleep duration and distribution of sleep over 24 hours are regulated by complex interconnected mechanisms, involving both the need for sleep accumulated during wakefulness -homeostatic process- and biological rhythm -mainly the circadian process- under the influence of external synchronizers like light-dark cycle. The propensity to sleep at a given moment is regulated by a physiological signal, sleepiness, in connection with the level of vigilance. The characteristics of sleep are relatively stable for a given individual because of a strong genetic determinism, but varies in duration and architecture according to age, individual habits, sleep schedules and environmental constraints. An understanding of the physiology of sleep is important for the clinician, allowing a better understanding of sleep dysfunction, responsible for various and frequent sleep disorders equiring appropriate care.