I love power napping, but you have to get it right. Ideally you want to have a 15-20 minute power nap during the early afternoon hours. The reason I say 15-20 minutes is that, your brain waves, will not start going in too deep sleep, but your sleep state will be low enough to refresh you. Once you get to 30+ minutes you have to start worrying about sleep inertia, the groggy, tired feeling you get once you wake up.
The way a normal sleep cycle works is about 90 minutes to 120 minutes, and In that you will generally go through 4 sessions of Non-REM sleep (Rapid Eye Movements) and one stage of REM sleep. It goes 1>2>3>2>R. Ideally you want to wake towards the end of that first session of stage 2 non-rem sleep, why because if you hit stage 3 you will go into deep sleep, waking up during deep sleep, will hit you with some pretty bad sleep inertia, that can take up to 30 minutes to recover from.
So keep your power napping sessions in the early afternoon, and between 15-20 minutes enjoy!