Caffeine actually stops the breakdown of energy so you have a surplus! In simple terms, ATP is what gives your body energy. Food is broken down and stored as ATP which is used as energy. When you start to feel yourself dragging, it is because you’ve used up all the ATP, because ATP cannot be stored for long term.
Caffeine blocks the signal in your body, adenosine, from binding and letting the body know you are out of energy. It tricks your body into thinking that there is energy there. The effects of the caffeine can kick in within 10 minutes and last up to 4-6 hours! That's why you crash from caffeine, when the effect runs out, your body tries to use ATP and there's none there!