The "Two-Process" Model of the Sleep-Wake Cycle

The upper, dark line in the graph ("S") represents the homeostatic sleep drive, currently thought to be mediated by adenosine metabolism. The lower, lighter curve ("C") represents the circadian rhythm, with rising and falling levels of arousal linked to the daily cycle of light and darkness.

As you can see, the homeostatic sleep drive rises steeply during waking hours—the longer we're awake, the stronger it gets. It sharply declines at sleep onset—the more we sleep, the weaker it gets.

The circadian rhythm has a slower, smoother curve, reflecting a gradual shift in arousal levels. Our arousal levels are highest during the day, and lowest during the night.