Figure 9

Representation of the two-process model of sleep regulation. Sleep propensity, reflected by the homeostatic Process S, builds up during wakefulness and declines during sleep (yellow). The two thresholds delimiting S are modulated by a circadian process (Process C) (blue). Sleep is initiated when Process S reaches the upper threshold (H) and is terminated when S reaches the lower threshold (L). A single circadian pacemaker (located in the suprachiasmatic nucleus [SCN]), entrained by an external zeitgeber, generates or synchronizes various physiological circadian oscillations. Rise rate (r) and decay rate (r1) of Process S are different. (Adapted from ref. 47)

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published 2000