Thanks for the link, Lela. Setting a timer is crucial because it lets you indulge while making sure that you also get that necessary wakeup call (uh, I've been guilty of letting 20 minutes turn into 45). It can be a little disconcerting at first, especially if you're constantly thinking about when your alarm is gonna go off ("has it been 15 minutes already?") but once you get past that initial neuroticism, you can get into the perfect snooze mode.
Isn't it funny how we tell ourselves we don't have time for a nap (or meditation or a walk or whatever), but then when we make time for it, we gain so much productivity? It's like giving 20 minutes to gain 60. Interesting how that works.
I consider myself the queen of midday napping. Sometimes when things are quiet at work, I find myself wanting to doze off, and sometimes I just have to give myself permission to do that. I try my best to clear my mind of anything that would even remotely stress me out, and, yes, making sure you're in the dark is very helpful.