A short nap washes out the adenosine, which allows the caffeine to bond to the same receptors so no new adenosine can get in.
Remember: caffeine is not a stimulant. It doesn't give you energy. It only stop your brain from noticing that you are tired.
Incidentally, this is why mixing alcohol and, say, Red Bull is a really bad idea. Because the alcohol will make you just as hammered, but you won't get any fatigue along with it because your body can't notice fatigue. So you're wired as shit, you think you have lots of energy, and your judgement is still impaired.