That’s why "light sleep, or even just some rest without sleep" is key to taking a satisfying power nap (also, avoiding bedhead). Twenty or 30 minutes is the number you're shooting for.
Set a schedule
Hitting the sack doesn't guarantee shutting off the brain -- sometimes no matter how many sheep you count, those fluffy buggers start turning into work emails, appointments, and all kinds of other fodder for being conscious and cranky.
But before forfeiting siestas for espresso, Dr. Penzel says to give napping on a fixed schedule a try. It doesn't matter if you take a nap on your break or first thing after getting home after work (well, second if you count taking off pants). Be consistent and your body will understand what to do.