Some companies have actually created napping rooms in their facilities in or allowing employees to nap at their desks. The reason why is simple. Rested workers are more alert, in a better mood, and concentrate better, as reported by William Anthony, author of The Art of Napping at Work. He has conducted a survey in which 70 percent of the more than 1000 respondents said that they nap at work.
My suggestion would be to experiment with meditation, as I believe it can leave you more refreshed and revived than taking a nap. I will do that every once in awhile when I need more energy late in the afternoon.