I wrote some time ago that taking a nap might be the best way to get refreshed during the day. Now the folks at lifehack.org write about how to do it right.
It seems that you want to aim for the end of your REM stage to wake up, before the deep sleep kicks in. Apparently, if you get the timing right, you can get it down to as little 15 minutes to get the most benefit. Check out the post and try it for yourself.