# Two processes decide when you are sleepy

> Sleep pressure builds while you are awake; the body clock runs regardless. How the two-process model explains the second wind and the costly late nap.

Source: https://dreamsheeps.space/learn/two-processes
Published: 2026-09-28 · Updated: 2026-10-14

Pressure builds while you are awake. The clock does not care about pressure.

Borbély's two-process model is the frame the rest of this fits into, and it has held up for four decades. Two systems, largely independent of each other.

Process S is sleep pressure: adenosine building from the moment you wake and discharging while you sleep. Process C is the circadian clock: a rhythm that runs whether you slept or not, producing a strong push toward alertness in the early evening and a deep trough in the small hours.

> You fall asleep easily when high pressure meets a falling clock. Miss the overlap and neither is enough on its own.

This explains the second wind. The circadian alerting signal peaks a couple of hours before habitual bedtime and can mask a great deal of accumulated pressure, which is why nine in the evening can feel wide awake after a genuinely bad night.

It also explains why a late-afternoon nap is expensive. Twenty minutes at five o'clock discharges pressure you were going to need at eleven, and the clock will not lend you any of its own.

## The longer answer

[Tired but can’t sleep](https://dreamsheeps.space/blog/tired-but-cant-sleep) applies the model to the evening it most often goes wrong: in the hours before your usual bedtime the body clock pushes hardest for wakefulness, however tired you are. [How long should it take to fall asleep?](https://dreamsheeps.space/blog/how-long-to-fall-asleep) gives the normal range, up to about 30 minutes, and [how to fall asleep fast](https://dreamsheeps.space/how-to-fall-asleep) what the research supports when it takes longer.

## Sources

1. [Borbély et al. — The two-process model of sleep regulation: a reappraisal](https://doi.org/10.1111/jsr.12371)

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