Two-Process Model of Sleep Regulation
The two processes of Alexander Borbély's two process model of sleep regulation (Hum Neurobiol 1:195–204, 1982). A propensity for sleep increases during wakefulness and dissipates during sleep. In the model this homeostatic sleep process was referred to as Process S, and its dynamics are derived from the sleep-wake dependent changes in electromyographic (EEG) slow-wave activity. Process S interacts with a circadian process, Process C. Process C defines an upper and a lower threshold between which Process S oscillates. Sleep onset is triggered when Process S reaches the upper threshold and sleep continues until Process S reaches the lower threshold. The thresholds were estimated on the relationship between sleep duration and circadian time at which sleep is initiated and on the dynamics of EEG slow wave activity (see Process S).