Abstract
Sleep homeostasis and circadian rhythmicity are two main processes governing both sleep propensity and sleep structure. Sleep homeostasis refers to the processes monitoring how long one has been awake, how long and how deep one has slept, and what sort of sleep one has obtained. Sleep-homeostatic processes ensure that on average one obtains sufficient sleep to restore those functions and variables, which are affected by wakefulness. Circadian rhythmicity, in the context of sleep regulation, refers to the near 24-h variation in the propensity to sleep. Circadian rhythmicity ensures that sleep occurs primarily at the appropriate time of day, which is during the nighttime in humans.