Competing for the receptor, not the molecule
Caffeine works because it resembles adenosine closely enough to fit the same A1 and A2A receptors. By occupying these sites without activating them the way adenosine does, caffeine blocks the calming, sleep-promoting signal that adenosine would otherwise deliver. This is competitive antagonism: the more adenosine present, the more caffeine is needed to hold the receptors.
Because the accumulated adenosine is still there, caffeine masks tiredness rather than erasing it. When the caffeine clears, the built-up sleep pressure can reassert itself, sometimes felt as a slump.
Timing, tolerance, and individual differences
Caffeine takes hours to clear, and the time varies widely between people depending on genetics, liver enzymes, and other factors, so an afternoon dose can still affect sleep that night. With regular use the brain can adjust receptor numbers, contributing to tolerance and to withdrawal symptoms when intake stops.
This is general educational information about how caffeine interacts with the brain, not advice on dosing or use; individual sensitivity varies and concerns are best discussed with a qualified professional.