You reach for coffee because it wakes you up. That's the whole premise, until you look at what caffeine is actually doing in your brain, which is closer to muting an alarm.
Caffeine occupies the receptor. Adenosine queues outside, unread.
01The alarm you're not hearing
Your brain keeps a running score of how tired you are, using a chemical called adenosine. It climbs steadily through the day, the longer you're awake. Caffeine is an adenosine receptor antagonist or in simple words, caffeine occupies the receptors adenosine would normally bind to, so the tiredness signal can't get through. You stop reading the alarm. It doesn't stop ringing.
Caffeine isn't inert once it's blocking that signal, either. It also has a real, smaller stimulant effect. So it gives you a genuine, if modest, mood lift. But that lift is layered on top of a masked signal, not a substitute for the tiredness actually going away.
02The debt keeps compounding
While you feel fine, the adenosine keeps accumulating underneath the block. Being awake and alert didn't make the tiredness disappear. It's still there, waiting behind a signal that's currently being muted. The gap between how tired you are and how tired you feel is doing nothing but growing.
03The crash is the alarm getting through
Caffeine's block doesn't last forever. As it clears from your system, the adenosine that built up all day finally reaches the receptors it couldn't reach before, all at once. That's the caffeine crash: not new fatigue, but the fatigue that was already there, arriving late.