What is Process S?

• A. The brain's circadian rhythm.
• B. The wakefulness drive regulated by light.
• C. The daily cortisol cycle.
• D. Homeostatic sleep drive/sleep pressure.

Answer: (D)

Process S is the homeostatic sleep drive, the pressure to sleep that builds up the longer you stay awake. As time awake extends, brain chemicals (notably adenosine) accumulate and push you toward sleep, making sleepiness greater. When you do sleep, this drive dissipates, and you wake up with the reset of wakefulness pressure for the next cycle. This sleep pressure interacts with Process C, the circadian rhythm, to shape when you feel alert or sleepy—Process S explains the buildup and relief of sleep pressure, while Process C governs the timing across the day. The other rhythms, like cortisol cycles or light-driven wakefulness, influence alertness but do not constitute the homeostatic sleep drive itself.