IN VIEW OF THE COMPLEXITY OF SLEEP, MODELING ITS MAJOR REGULATORY PROCESSES WAS AN ESSEN-

to stimulate new experiments and interpret the results. The two-process model1,2 postulates the interaction between a homeo-static Process S and a circadian Process C that is independent of sleep and waking. The homeostatic Process S takes into account the prior history of sleep and waking. Process S rises and declines monotonically during waking and sleep, respectively. Its increase and decline were described by exponential functions.2 The saturating exponential increase of S during waking was first derived from sleep deprivation data2 and subsequently confirmed by a multiple nap protocol.3,4 Slow-wave activity (SWA; EEG power in the 0.75–4.5 Hz range), a marker of sleep intensity, was used to determine the time course of Process S during sleep.1,2,5,6 Parameter estimation in individuals is difficult. Therefore, pa-rameters of the decline of Process S were derived from averag