The three-process model of alertness, explained
Updated October 8, 2026 · 2 min read
The three-process model of alertness predicts how alert a typical person will be from time awake and prior sleep (process S), the body clock (process C) and grogginess after waking (process W). It sums them into one score on which 14 means high alertness, 7 is the sleepiness threshold and 3 is extreme sleepiness.
The three processes
| Process | Tracks | How it moves |
|---|---|---|
| S, sleep pressure | Time awake and prior sleep | Falls quickly after waking, then levels off; climbs back during sleep, usually fully within 8 hours |
| C, body clock | Time of day | A 24-hour wave, lowest near 05:00 and highest near 17:00 |
| W, sleep inertia | The minutes after waking | A brief dip on waking that fades |
The model was first presented in 1987 and later extended to predict sleep. The published version adds S and C and leaves W out, and Capillary's calculator does the same. A later term, U, adds a small afternoon dip.
Reading the score
The scale normally runs from 1 to 21. In practice its authors read 14 as high alertness, 7 as the sleepiness threshold and 3 as extreme sleepiness. They tied 7 to slow rolling eye movements, a physical sign of sleepiness.
In their validation studies, predicted scores of 8 to 14 came with little change in brain-wave alpha activity. Below 7, alpha rose sharply, sometimes doubling.
To compare with the Karolinska Sleepiness Scale, a 2014 validation study converts the score as 10.6 minus 0.6 times alertness. A score of 7 is about KSS 6.4, and a score of 5 is about KSS 7.6.
Worked example: day, evening and night
Take three single shifts, each after sleep from 23:00 (to 06:00 before the day shift, to 07:00 otherwise). Capillary's shift alertness calculator, which runs the model, predicts these lows:
| Shift | Lowest | When | Below 7? |
|---|---|---|---|
| Day, 07:00 to 15:00 | 10.9 | 07:00 | No |
| Evening, 15:00 to 23:00 | 8.7 | 23:00 | No |
| Night, 23:00 to 07:00 | 5.0 | 05:45 | Yes, from about 01:15 |
Only the night crosses the line. By 05:45 the person has been awake almost 23 hours and the body clock is near its low, so both terms pull down together.
How the model fills in sleep
The model can run from a work schedule alone, so it has to assume when people sleep. The published rules include:
- Bedtime at 23:00 on ordinary nights.
- After a night shift, bed 1 hour after the shift ends.
- Sleep that starts between morning and noon lasts about 4.5 hours.
- Morning and evening types have the body clock moved 1 or 2 hours earlier or later.
- A habitual sleep need of 8 hours.
How well it works
A 2014 study followed 136 airline crew members who logged sleep and sleepiness on a phone or tablet app. The model's predictions tracked their ratings. Sleep pressure plus the body clock fit best, especially with the clock adjusted by one question about morning or evening type, and adding the sleep-inertia term made the fit worse.
The model describes a typical person, built from group averages. It does not see how you slept, caffeine, light, physical activity or workload, so use it to compare schedules rather than to judge one person's state.
Common questions
- Is it the same as the two-process model of sleep?
- No, but the two are close relatives built on sleep pressure and the body clock. The three-process model's authors note that its parameters are numerically very close to the two-process model's, and the 2014 study suggested those two processes do most of the work.
- Who uses it?
- In 2004 its authors reported it in use for evaluating navy, airline and railway work schedules, and for teaching how sleep and wakefulness are regulated.
- Why is the low near 05:00?
- The body-clock term bottoms out at about 04:48 in the model. Its authors set it from studies in which alertness and performance were lowest in the early morning, around 05:00.
See your own shifts this way.
Bring in your schedule from any scheduling app with its calendar link. Capillary charts your hours, nights, quick returns and hardest shifts. Only you can see it.