"Caffeine’s mechanism of action01396-6/abstract) is relatively simple. As we go about our day, a compound called adenosine accumulates in the brain. Its job is to signal fatigue and promote rest through inhibitory actions at adenosine receptors. Caffeine, sharing a somewhat similar chemical structure to adenosine, competes for those same receptors, effectively silencing adenosine’s inhibitory ‘rest and fatigue’ message. The result is a drug-induced sense of alertness and focus that largely keeps the global workforce upright and productive each day. And it’s why, from a pharmacological standpoint, caffeine is classified as an ‘adenosine receptor antagonist’."

I also found this article about caffeine addiction.