Why do you feel sleepy at some times of day and wide awake at others, even after the same night's sleep? The most influential answer is the Two-Process Model of Sleep Regulation, proposed by Swiss sleep researcher Alexander Borbély in 1982. It describes sleepiness as the interaction of two separate processes: sleep pressure that builds the longer you're awake, and a body clock that runs on a roughly 24-hour cycle.
Process S vs. Process C#
Borbély proposed that sleep and wakefulness are governed by two independent, interacting processes.
1. Process S: The Homeostatic Sleep Drive
Process S is sleep pressure. It rises the whole time you're awake and falls while you sleep. One chemical signal behind it is adenosine, a by-product of the energy your neurons use.
- As adenosine accumulates, it binds to receptors (mainly A1 and A2A) that damp down wake-promoting brain activity.
- In the model, Process S rises along a curve that gradually flattens during the day, then drains away during sleep. By early afternoon, sleep pressure is well above its morning level, and it keeps rising until you sleep.
2. Process C: The Circadian Pacemaker
Process C is the body clock, generated by the suprachiasmatic nucleus (SCN), a small cluster of neurons in the hypothalamus, and kept in step with the day mainly by light.
- Process C doesn't track how long you've been awake; it runs on its own roughly 24-hour cycle.
- Its alerting signal rises through the day and peaks in the evening, which is what holds rising sleep pressure off until bedtime.
- Many people also feel a smaller dip in alertness in the early afternoon. Why exactly it happens is still debated, and it shows up whether or not you've eaten lunch.
What the Model Predicts#
Put the two curves together and several everyday experiences fall out of it:
- Bedtime: You fall asleep most easily when sleep pressure is high and the circadian alerting signal is dropping, late in your biological evening.
- The evening second wind: A few hours before your usual bedtime, the circadian signal is at its strongest. Many people feel oddly alert then; researchers call it the wake maintenance zone.
- The afternoon dip: By early afternoon, sleep pressure has been building for hours while the circadian signal briefly levels off, so drowsiness gets through.
- Jet lag and night shifts: When you try to sleep at the wrong biological time, the two processes are out of step. Pressure says sleep, the clock says wake, and sleep comes out short and light.
2 PM is the textbook time for that dip; yours may be different. ARC asks for one-tap energy check-ins, and after about ten days (roughly four afternoon check-ins) it measures when your dip really lands and moves your plan to match. For practical ways to soften it, see why you crash at 2 PM.
Caffeine blocks adenosine receptors for a while, but it does nothing to reduce Process S. Adenosine keeps building underneath, so the tiredness comes back once the caffeine wears off.
What You Can Change#
The model points to a few levers:
- 1.Sleep is the only thing that drains Process S. A short nap (20 minutes or less) takes a little off without making it hard to sleep that night; see the science of naps.
- 2.Light strengthens Process C. Morning daylight keeps your clock on time, and bright light has an immediate alerting effect that can blunt the afternoon dip. A short walk outside after lunch does both.
- 3.Caffeine timing. Caffeine masks Process S rather than lowering it, so the question is when it wears off. There's no universal cutoff: for one average cup and an average metaboliser, caffeine takes roughly five hours to fall below 50 mg, and a bigger cup, a slower metabolism or an earlier coffee still in your system pushes that earlier. The caffeine half-life guide goes through the math.
- 4.Lunch. A large, high-carbohydrate lunch can make the dip feel deeper for some people, but the dip happens without lunch too.
Delaying your first coffee until 60 to 90 minutes after waking is a popular recommendation; the evidence is mixed, and the best test is your own afternoons.
The model has been refined since 1982 (Borbély and colleagues published a reappraisal in 2016 in the Journal of Sleep Research), but its core idea still underpins much of sleep science. For how adenosine builds and clears in more detail, read the adenosine and sleep pressure guide.
Educational content, not medical advice. ARC is a wellness app, not a medical device.