Sleep glossary
Two-process model
Sleep pressure and the body clock as two independent systems, and why understanding that they are separate explains most confusing things about tiredness.
In short
The two-process model explains sleep timing as two independent systems: sleep pressure that builds the longer you are awake, and a circadian rhythm that rises and falls on a roughly 24 hour cycle. How sleepy you feel is the gap between them.
Two-process model is also called Process S and Process C or Borbely model.
The model is the standard framework for thinking about sleep timing, and its usefulness is that it separates two things people habitually merge. The first is sleep pressure, often called Process S, which builds steadily from the moment you wake and discharges while you sleep. The second is the circadian rhythm, Process C, an internal oscillation running on roughly a 24 hour period that does not care how long you have been awake.
How sleepy you feel at any moment is the interaction between them. That is why a single number for tiredness never explains a real day: you can be carrying a lot of sleep pressure and feel alert because the circadian signal is peaking, or be well rested and feel dreadful because it is in a trough.
What the model explains that nothing else does
Three common experiences fall straight out of it. The afternoon dip is a circadian trough arriving while pressure is only moderate. The second wind at eleven at night is the wake maintenance zone, a stretch of strong circadian wake drive that can override several hours of accumulated pressure, which is why pushing past your bedtime often makes falling asleep harder rather than easier. And the reason a nap can wreck a night is that it discharges pressure without touching the clock, so the pressure is missing at bedtime while the timing signal is unchanged.
It also explains why the two levers for fixing a schedule are different tools. Sleep pressure responds to how long you have been awake, so it is adjusted by changing sleep and nap timing. The clock responds to zeitgebers, overwhelmingly light. Trying to fix a circadian problem by being more tired does not work, and it is the most common mistake in sleep advice.
How to fix your sleep schedule applies both levers in the right order, and nap lengths covers the pressure side of the trade.
Questions people ask
What are Process S and Process C?
Process S is the homeostatic sleep drive, which builds steadily the longer you are awake and discharges while you sleep. Process C is the circadian process, an internal rhythm that rises and falls on a roughly 24 hour cycle regardless of how long you have been up.
Why am I tired all afternoon but wide awake at 11pm?
Because the two processes move independently. Sleep pressure keeps climbing through the evening while the circadian signal produces a strong burst of wake drive in the late evening, and that burst can more than cancel the pressure for a couple of hours.