The two-process model: Origin and perspective

In the two-process model as developed in the early 1980's sleep is controlled by a process-S, representing the rise and fall of sleep demand resulting from prior sleep-wake history, interacting with a process-C representing circadian variation in sleep propensity. S and C together optimize sleep timing and intensity with respect to internal demand and external opportunity. The theoretical distinction between S and C has been useful in understanding many aspects of sleep, most prominently 'internal desynchronization' and circadian vs. homeostatic aspects of physiology. Both S and C concepts met difficulties as they were further explored in mathematical detail. The simple S dynamics of the model became problematic when SWA dynamics appeared to vary between brain areas. The C process was originally identified with the circadian pacemaker in the SCN. Some researchers even specifically attributed the direct control of activity and rest to the SCN. Yet, activity-rest rhythms under special circumstances (feeding schedules; methamphetamine) turned out to require neither the SCN nor circadian clock genes, and have an oscillatory basis themselves. Recent studies show that in animals the activity-rest cycle adaptively shifts from night to day under metabolic influence. The SCN itself remains rigidly tied to the day and night outside, acting as a clock entraining, not dictating other circadian processes. The original S-C model provides a basic framework for the much needed analysis of the role of metabolism in the temporal organisation of sleep and wake. It will require adjustment, not abandonment