Caffeine bonds to the same receptor sites as adenosine, acting as a competitive antagonist. Adenosine is a neuromodulator that binds to adenosine receptors in the brain to signal drowsiness and initiate sleep.
When the adenosine receptor sites are occupied by caffeine, new adenosine receptor sites are opened up, allowing the adenosine to be received, initiating the sleep sequence.
The next time the person consumes caffeine the same cycle repeats, producing more adenosine receptor sites. This is why it takes more and more caffeine to get the same level of alertness over time. This is the mechanism that explains caffeine "tolerance".
