Have you ever woken up from a nap and felt WORSE than when you started? Dr. Jeffrey Iliff (Eye-Liff) is a sleep researcher, and a clip of him explaining how to take the perfect nap is making the rounds. He gives you two choices.
If you take a short nap, say 20 or 30 minutes, you won't go into a deep sleep, and you'll wake up feeling refreshed. If you want to go longer, nap for 90 minutes. That completes one full sleep cycle, and again, you'll wake up feeling recharged.