Process S — The Science Name for Sleep Drive

In the classic two-process model of sleep, Process S is the homeostatic sleep drive that grows with time awake and drops with time asleep. The longer you’ve been up, the higher Process S climbs; when you finally sleep, it falls back down. This is what we unpack more conversationally as sleep drive.

Process S interacts with Process C — your circadian rhythm. You sleep best when both the drive to sleep (S) is high and your internal clock (C) is in its “night” phase. When those are misaligned (jet lag, late naps, erratic schedules), you can feel sleepy at the wrong times and wired when you’d like to be out cold.

For insomnia, Process S shows up in a few key ways. Long naps or sleeping in shrink the gap between S and C at night. Spending lots of time in bed awake lets S leak away without anchored clock signals. CBT‑I uses this model under the hood when it tightens time in bed and locks in a consistent wake time — letting Process S build strongly enough that sleep becomes more automatic again.

You never see Process S directly, but you feel it as the difference between “I could sleep” and “I could fall asleep on this keyboard.” Most of the helpful advice around wake times, naps, and caffeine boils down to respecting this curve instead of constantly tugging it in opposite directions.

Why It Matters

Putting a name and a model to sleep drive helps turn trial‑and‑error into targeted experiments — so you can adjust your day in ways that measurably change your nights.

Closing Line

Your brain is tracking Process S whether you know it or not; working with it means you finally stop gaslighting your own sleepiness.