I work as a contractor at Google MTV. My building has a very large lounge area and I sometimes see people taking naps on the couches. I work from 6:30am to 4pm (beat the traffic!) and I use this room not for naps, but to go read for 10 minutes or so to unwind. Long walks also help me stay fresh. In any case, I have always believed that frequent breaks are important. A bit off topic, but when I work at home (which I have mostly done for the last 15 years) then I almost always take a 20 minute nap after lunch.