Caffeine reduces sleepiness by preventing adenosine from being read at A1 and A2A receptors. Adenosine itself is unchanged, so the sleep drive is masked rather than removed.
Two different operations
Removing the signal
- Adenosine would be cleared or its production stopped
- Sleep drive would fall at its source
- Effect would persist regardless of how long you stay awake
Blocking the signal (what caffeine does)
- Adenosine stays at its current concentration
- Sleep drive keeps accumulating underneath
- Effect weakens as adenosine outcompetes the same dose
Why the same cup fades by afternoon
Competitive antagonism is surmountable. Caffeine and adenosine occupy the same site, so the fraction of receptors blocked depends on the balance between their concentrations and affinities. As adenosine rises across a long waking period, it wins back more of the sites, and a fixed caffeine dose blocks proportionally fewer of them. The drug has not been metabolized away yet and the adenosine has not vanished — the ratio has moved.
This page covers the acute competition only. How fast caffeine is cleared from the body, and how the receptor population itself changes with repeated daily use, are separate mechanisms treated in later chapters.