Slow-wave sleep in daytime and nocturnal sleep: An estimate of the time course of "Process S

Abstract Daan et al. (1984) have proposed that sleep and wakefulness are regulated, in part, by a "Process S " that increases during wakefulness and declines during sleep. Data derived from several studies were taken to determine the time course of Process S during both wake-fulness and sleep. As required by the model, slow-wave-sleep (SWS; an index of Process S) was found to increase exponentially as a function of prior wake time (equation 1) and to decline exponentially as a function of time asleep (equation 2). The equations accounted for 91 % and 96 % of the variance, respectively. In addition, equation 1 accurately predicted the amount the amount of SWS in the first hour of nocturnal sleep. Borb6ly (1982) and Daan et al. (1984) have proposed that the timing and duration of sleep are determined by two processes. One, called &dquo;Process S,&dquo; increases during wakefulness and decreases during sleep; the other, called &dquo;Process C,&dquo; is controlled by a circadian oscillator that is unaffected by sleep and wakefulness, and defines the upper (H) and lower (L) thresholds in the model (see Fig. 1). According to Daan et al. (1984), &dquo;S increases monotonically during wakefulness until it reaches H, at which point sleep is initiated. S declines monotonically during sleep until it reaches L, when sleep is terminated&dquo; (pp. R162-R163). FIGURE 1. Schematic representation of the two-process model of sleep regulation (after Daan et al., 1984). Process S fluctuates between upper (H) lower (L) thresholds determined by a circadian Process C