Commentary on the three-process model of alertness and broader modeling issues.

1. A sinusoidal circadian process (Process C, sometimes considered as two separately modifiable components, Process C during wake and Process C during sleep); 2. A homeostatic process that declines exponentially toward a minimum during wake (Process S) and increases exponentially toward a maximum during sleep (Process S ); and 3. A sleep inertia process, which results in depressed alertness on awakening, and then exponentially improves toward...