Doesn't that mean you're napping for too long? A short nap (20 minutes) is supposed to be very refreshing because you don't go into deep sleep. Deep sleep is hard to wake up from and you feel groggy when you do.
Sometimes I have to take a nap in the late afternoon. I feel as if I'm about to become unconscious if I don't sleep. It's a very weird feeling, but I try not to sleep for too long, although it's very hard to wake up before going into stages 3 and 4 (deep sleep). I should be setting an alarm clock I guess.
It's even like that when I only sleep for an hour, so I usually don't even bother setting an alarm. I figure if I'm going to feel like crap, I might as well make it worth my while. And if I set an alarm for less than an hour, I find it hard to fall asleep because I'm focusing so hard on trying to do it quickly so I don't waste most of the time I set aside for sleeping. So it's kind of a no-win. Even when I take a 2-3 hour nap, I wake up naturally (never set an alarm for those) and feel like crap - would I wake up naturally straight from deep sleep?
I dunno, I just seem like I'm not cut out for it!