The present paper gives a short overview on what is known about human sleep/wake regulation and focuses on two main component, the homeostatic component and the circadian. The homeostatic components increases sleep need exponentially as a function of prior wakefulness whereas the circadian component is a consequence of the 24-hour physiological rhythm facilitating sleep during the night and counteracting sleep during the day. Normally, these components interact, in an additive way. This interaction has been described in mathematical models that can be used to predict sleep duration, depth of sleep and the level of alertness given the knowledge of circadian phase and prior time awake.