TIMING OF SLEEP MODELING - CIRCADIAN MODULATION OF THE HOMEOSTATIC PROCESS

Some basic assumptions of the two-process model (somnostat; Daan et al . 1984) were used to develop a more general model called the rhythmost at. It addresses the timing mechanisms underlying the sleep-wake cycle (somnostat). One of the two main features of rhythmostatic regulation is illustrated by a mathematical model of the relay thermostat. Like the time course of room temperature is representing the process of hea t regulation in the relay thermostat, slow-wave activity is representi ng sleep-regulating processes in the somnostat. The observed alternati ons of reverse-exponential and exponential phases in the day-and night time course of slow-wave activity reflect the deviations of a regulate d process around a set-point level. These oscillations may result from time delays in the process of sleep regulation. However, unlike the p arameters in the relay thermostat model, the parameters of the regulat ed process in the somnostat (e.g. set-point and time constants for rev erse-exponential and exponential phases) are modulated by the circadia n pacemaker. To account for the second main feature of the rhythmostat ic regulation, a circadian term (a sine function with 24-h period) was incorporated in the mathematical model describing the time course of slow-wave activity. The data of sleep deprivation experiments of Dijk et al. (1987b, 1990, 1991), and Akerstedt and Gillberg (1981, 1986) we re used to estimate the human somnostat parameters. The simulations of these data offer explanations for some phenomena which can not be exp lained by the model of Daan et al. (1984).