Sleep homeostasis in the rat: Simulation of the time course of EEG slow-wave activity

Abstract

According to the two-process model of sleep regulation, a homeostatic Process S increases during waking and declines during sleep. For humans, the time course of S has been derived from the changes in EEG slow-wave activity (SWA; spectral power density in the 0.75–4.0 Hz range) during sleep. We tested the applicability of the model to sleep in the rat. The simulation was based on the vigilance states for consecutive 8-s epochs of a 96-h experiment in 9 animals. The level of S was made to decrease in epochs of non-REM sleep (NREMS), and to increase in epochs of waking or REM sleep according to exponential functions. By optimizing the initial value and the time constants of S, a close fit between the hourly values of SWA in NREMS and of S was obtained. The biphasic time course of SWA during baseline, its enhancement in the initial recovery period after 24-h sleep deprivation, and its subsequent prolonged undershoot were present in the simulation. We conclude that sleep homeostasis as conceptualized in the two-process model may be a general property of mammalian sleep.