When this thing that requires some actual thought and work isn't related to something I'd otherwise have to do anyway? Usually, I tell myself to do it anyway. If it happens to be an extension of whatever I'm doing at the time, I don't mind too much. I tend not to feel burned out if I have to do some long stretch of work. Exception: late-night stuff on saturday (like today! :D), in which case I'm almost always going to put off *insert thing I ought to be doing but won't because I won't feel the impact of not doing it until later*.
Sometimes, though, I just don't want to open that notebook laying there on the oh-so-comfy-looking bed, and, hey, that incomplete code is just sitting around on the laptop screen so if I close the laptop that textbook would fit neatly on top of it and the bed would be free... so I tuck away whatever would otherwise have priority and take a nap. (This is more of an issue of a lingering "problem"/benefit with a sleep experiment I put myself through rather than not liking the stuff I'm doing... but that's irrelevant atm :P.) I usually feel energetic after half an hour or so.
*edit* eh, I experimented w/ other sleep cycles before. I ended up going back to that 8hr thing everyone else does but got stuck w/ a noon nap. (part time work + classes left a small window of opportunity open, convincing me to foolishly pursue that sleep cycle test... meh. didn't work out so well.) Anyway, this relates to the above in that, for the month I actually kept up a modified sleep cycle, I felt totally fine with 6 hrs (while feeling more, eh, "active" and attentive). So, I'd recommend that if you happen to have a schedule that allows for weird nap times and want to get yourself in the mood to get more "stuff" done.
