Which two processes regulate sleep and wakefulness?

• A. Process C and Melatonin
• B. Process S and Cortisol
• C. Process C and Process S
• D. Melatonin and Cortisol

Answer: (C)

The thing being tested is how sleep and wakefulness are regulated by two interacting processes. One is Process S, the homeostatic sleep pressure that builds up during wakefulness and dissipates during sleep, determining when you feel compelled to sleep and how deep that sleep will be. The other is Process C, the circadian rhythm that cycles roughly every 24 hours and sets the timing of wakefulness and sleep relative to day and night, guided by light cues to the brain’s master clock. Together, these two processes explain why you get sleepy after staying up a long time and why you tend to wake up around a regular morning time, as well as how daylight and darkness influence alertness and sleep duration.

Melatonin and cortisol are important signals that reflect circadian phase and arousal, but they are not the two regulating processes themselves. Melatonin promotes sleep as night falls, and cortisol tends to rise in the morning to help wake you up; they are outputs of the system and modulators rather than the fundamental regulatory processes described in the two-process model.