Time off is very important. Sitting at your computer, working 16 hours a day, 7 days a week is the express train to BurnOutVille (Population: You).
Let your brain rest and let your subsconcious chew on your code for a while (even if it's only a few hours a day). You need that time to enable your mind to take a step back and see the bigger picture of the project.
Your productivity will be higher and what you produce will be of higher quality.
I'm not sure I agree with no computer whatsoever -- just not coding. Play games, lazily surf the web, or whatever. Just don't write code. That works for me.