2. Energy:
Most people can see how taking a short siesta in the middle of the day would help their energy levels. It gives the brain a nice rest from processing information and decision making.
All the little neurons, axons, and dendrites get a chance to slow down and take a load off for a little while. One can imagine how much more powered up a person would be when getting back to the grind (or whatever it is you are working on) after such a rest.
The mitochondria is the part within a neuron that turns substance into energy for the neuron. This nap break can give it time to catch up on producing energy so that you come back prepared to take on it all with greater vitality.