Whether an individual is awake or asleep depends on the balance of forces promoting and inhibiting each of these two states .
At times the balance can be almost equal and the subject may begin to fall asleep if he or she had previously been awake, or to lighten from sleep if previously asleep. The mechanisms determining whether sleep or wakefulness predominates are incompletely understood, but three processes interact with each other and with circadian rhythms (
NREM sleep homeostatic (intrinsic) drive (process S)
This drive to enter sleep increases, probably exponentially, with the duration since the end of the previous episode of NREM sleep. It builds up during wakefulness and probably in REM sleep as well. The duration of NREM sleep in each sleep cycle is inversely related to the duration of REM sleep in the previous cycle.
This drive declines once NREM sleep has been initiated, again probably exponentially. Its time constant is short at around 150min so that after a few days of partial sleep deprivation a steady level of sleep drive is attained.
The homeostatic drive reinforces the cyclical nature of sleep and wakefulness and is analogous to other physiological needs such as hunger or thirst. The increased need to sleep according to the length of time awake is similar to the increase in hunger as the duration of abstinence from food increases.
An increase in homeostatic drive increases the duration and depth of NREM sleep at the expense of REM sleep. There is an increased duration and depth of NREM sleep (NREM sleep rebound) on the first night after sleep deprivation, but once the NREM sleep debt has been repaid after one or two nights, REM sleep duration increases.