Power Sleep, by Cornell sleep researcher Dr. James Maas, transformed my idea of naps. I highly recommend the book. He recommends short "power naps" to quickly get in some rest. The key is to wake up before your body goes into a deep sleep cycle. For me, this means approx. 15-20 minute naps when needed. It truly is astounding how much this rejuvenates you. These naps were insanely helpful back when I was in college.
That doesn't work for everyone. When I let myself sleep on the train, I tend to feel like a perma-zombie when I get off. I think it has something to do with the fact that I wake up at every stop, or stay halfway conscious so as not to miss my stop and therefore don't have a clear delineation between being awake and asleep (and getting off the train doesn't snap me out of it).
Buy on Amazon: http://amzn.to/bvumKg
WorldCat (library catalog search): http://bit.ly/c3NMpF
I just wish naps were more accepted in the corporate world. When I have a startup again, I'd love to have a "nap" room for employees. Google has sleep pods: http://www.businessinsider.com/google-sleep-pods-2010-6