I find a huge difference between a nap that lasts 30 minutes vs. a nap that lasts longer than that. If I sleep 20 to 30 minutes, I will need just about 5 minutes to wake up and I'll feel refreshed and ready to go. Let it go over 30 minutes, and I am gone for two hours, and then it takes an hour to wake up. I think I get into a much heavier level of sleep cycle with a nap that goes more than half an hour, and it's very difficult to shake off.
I have a friend who is a doctor who can actually power nap for 10 minutes in his office! I envy this!!