The two processes: sleep pressure and the internal clock
The model that explains why you flag at 3 pm and are wide awake at 11.
The two-process model
Your sleep is driven by two independent systems that add together.
Process S, homeostatic pressure
From the moment you wake, adenosine builds up in the brain. It is a by-product of energy metabolism, and its concentration rises with time spent awake.
The higher it climbs, the sleepier you get. Sleep is what clears it.
Caffeine does not remove adenosine: it occupies its receptors, which masks the sleepiness. Adenosine keeps accumulating. When the caffeine has been cleared, the accumulated pressure shows up all at once: that is the crash.
Process C, the circadian rhythm
An internal clock of roughly twenty-four hours and twelve minutes, located in the suprachiasmatic nucleus, which oscillates independently of how tired you are.
The rest is part of the Sleep & Energy module
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