The sleep of 4 groups (N = 8) of young healthy subjects was recorded during naps at either 09.00, 14.00, 19.00 or 24.00 h. For the first hour of sleep, conventionally scored slow wave sleep (SWS) and computed EEG power density (0-25 Hz) were found to increase exponentially as a function of prior wakefulness (range 3-18 h). The equations based on SWS and spectral analysis of the sleep EEG gave values for r' (the growth rate of the function) that were similar to each other (0.038 and 0.039, respectively) and to that reported by Daan et al. (1984; 0.055). These data are consistent with the proposal that process S, 1 of 2 processes believed to regulate sleep and wakefulness, increases exponentially during the waking day (Daan et al. 1984). In addition, they indicate that in this context SWS and spectral analysis provide measures of slow wave activity that are equally sensitive to changes in prior wakefulness.