The Two-Process Model: How Sleep Drive and Circadian Rhythms Control Sleep
What Controls When You Feel Sleepy and When You Feel Alert?
Sleep is controlled by two independent but interacting processes: Process S (sleep homeostasis), which builds sleep pressure during wakefulness and dissipates during sleep, and Process C (circadian process), which regulates the timing of sleepiness and alertness across the 24-hour cycle. This two-process model, developed by Alexander Borbély, revolutionized sleep science by explaining how sleep drive accumulates the longer you stay awake while circadian rhythms determine when you naturally feel most sleepy or alert, regardless of how long you’ve been awake.