I have been power napping for years after reading about it in a book called Quantum Fitness. It recommended a nap from 15 to 45 mins b/w noon and 3 pm only. I have found a 30 min nap to be best for me (I'm now a septuagenarian), and like Jody says, if you can get to that just dozed off moment, you are refreshed.
Awesome!
RC Anderson