The two-process model of sleep regulation postulates that a homeostatic and a circadian process underlie sleep regulation. The timing of sleep and waking is accounted for by the interaction of these two processes. The assumptions of two separate processes or of a single process resulting from their additive interaction are mathematically equivalent but conceptually different. Based on an additive interaction, subjective alertness ratings in a forced desynchrony protocol and subjective sleepiness ratings in a photoperiod experiment were simulated. The correspondence between empirical and simulated data supports the basic assumption of the model. a homeostatic process, a circadian process, and a proces-srepresentin~ sleep inertia (Folkard and •kerstedt 1987(Folkard and •kerstedt , 1989(Folkard and •kerstedt , 1992;; Akerstedt and Folkard 1990). In recent efforts to integrate various models, it was shown that they are not mutually exclusive but can be used as 'modules' of a composite model (Achermann and Borbrly 1992;Massaquoi and McCarley 1992).
In the following, we will reconsider the conceptual basis of the two-process model and present the results of simulating alertness and sleepiness data that had been obtained in two recent experiments: a forced desynchrony protocol and a shortened photoperiod schedule.