Caffeine is the world's most consumed drug. And why not? It has no long-term downsides, re-allocates your energy for when you need it, and is hardly ever deadly. All in all, caffeine is just pretty awesome.
Caffeine just makes you very active. You feel the tiredness disappearing, your reactions are suddenly super fast, and you feel the need to move. In short, don't drink when you want to sleep.