Instead, caffeine is an antagonist at adenosine receptors (just as Narcan is an antagonist at opiate receptors). Adenosine may be familiar in its role in adenosine triphosphate, or ATP, a primary source of energy. But adenosine also serves as a neurotransmitter and is thought to build up over the day, accumulating in synapses where it binds to its receptors, precipitating a state of drowsiness. When caffeine is on board, adenosine signaling is blocked and as a result temporarily prevents or relieves drowsiness and maintains or restores alertness.
