Last weekend I took the entire weekend off guilt-free, because I had absolutely nothing to do; my e-mail inboxes were empty, any research tasks were in the hands of collaborators, and I was three weeks ahead in my lab planning. I had a normal life for two and a half days.
This weekend, I still kind of took the weekend off, but with plenty of guilt. There were any number of things I could have been doing, should have been doing, but I was seduced by a sale on the "Chuck" Season 1 DVD, and couldn't quite pry myself away from my TV. I still got something (one, tiny thing) done, and I'm not behind - but I'm not ahead any more, either.
Much as I said in my response to ScienceWoman's poll on taking time off between semesters, and in the graduate school survival kit I gave to a friend, I refuse to stress out about my failure to get work done. I have no home life, and can make up for it by just working 10- to 15-hour weekdays. And even if I don't make it up, there's no point fretting about it now.
Still, I do need to develop a strategy for getting work done on days when I have no meetings or classes or any reason to leave the apartment. That's the downside to a studio apartment; it's impossible to have one productivity area set aside.