Nobody can work 24/7 without needing a break. If you ever feel run down and have worked hard for a number of days, take a break. You deserve it.
Let's say you've cranked out 15-20 pages a day for the last two weeks. You get to page 250, right at the beginning of what could be the third act of your novel, and you're exhausted. Take the night off. Rent a movie. Go for a drive and take your significant other out to dinner. Celebrate.
Breaks are necessary or else you'll burn-out.
You could even try daily naps. Michael Hyatt recently blogged as well as had a podcast on the secret power of naps. Definitely worth a read and listen. Now, he didn't tell you to take a 2 hour nap, but a 15-20 minute power nap. On the weekends, I find myself wanting to nap for a few minutes in the afternoon. Sometimes I do it, sometimes I don't.
What do you do for breaks?