How sleepy you feel comes from two forces working together. This is the two-process model described by Alexander Borbély. Process S is sleep pressure. It builds while you are awake as adenosine collects in the brain, and sleep clears it. Process C is the circadian clock, which sends a wake signal that rises and falls each day. Set your schedule, then add coffee or an all-nighter and watch two days unfold.
Two days, hour by hour
Sleep pressure (S)Clock wake signal (C)How sleepy you feelAsleep
Sleep pressure-
Clock signal-
You feel-
Your schedulechange it and watch the graph
This is a simple teaching model with made-up units. It shows the shape of the ideas, not exact values for any one person.