Sleepiness is not simply the absence of stimulation. It is driven by a measurable process in which a byproduct of brain activity accumulates during waking hours and is cleared only by sleeping.
Two systems determine when sleep happens
Sleep timing is governed by the interaction of a homeostatic drive that builds with time awake and a circadian rhythm that promotes alertness at particular hours.
The homeostatic component is often called sleep pressure. It rises steadily from waking and dissipates during sleep.
Sleep occurs readily when pressure is high and the circadian signal is permissive, which is why the same level of tiredness feels different at different times of day.
Adenosine is the accumulating signal
Brain cells consume energy continuously, and adenosine is released as a byproduct of that consumption.
Adenosine binds to receptors that dampen the activity of arousal-promoting neurons, so its accumulation progressively reduces the brain's capacity to sustain wakefulness.
During sleep, adenosine is cleared, which is why pressure is lowest immediately after a full night and highest after prolonged waking.
Caffeine blocks the signal without clearing it
Caffeine occupies adenosine receptors without activating them, preventing the sleepiness message from being received.
The adenosine itself continues to accumulate throughout. Nothing has been reversed, only masked, which is why the tiredness returns as the caffeine is metabolised.
Because clearance takes several hours and varies considerably between individuals, an afternoon dose can still be occupying receptors at bedtime.
Naps reduce pressure as well as restoring alertness
A daytime nap clears some accumulated adenosine, which restores alertness and simultaneously lowers the drive to sleep that evening.
This is why a long or late nap can make falling asleep at night harder, while a short early one generally does not.
Waking from deep sleep produces a period of grogginess called sleep inertia, which is one reason nap length affects how useful the nap feels.
Why lost sleep is only partly recovered
After deprivation, the following night shows a rebound with a greater proportion of deep sleep, which clears pressure faster than normal sleep does.
That rebound is efficient but incomplete, so a weekend of extra sleep does not fully reverse a week of restriction on measures of attention and reaction time.
Persistent daytime sleepiness despite adequate opportunity to sleep is a reason to seek clinical assessment rather than to adjust caffeine intake.