Breaks cost me dearly.
Yesterday I had to do zero writing and go see the doctor (medical insurance is running out), and today writing was like pulling teeth. And hair. And guts. Slow, dull, hazy. I couldn't find the right words, the flow just wasn't there.
I used to break for the weekends, but Mondays were torture. So I stopped. I now write though weekends, holidays, what have you. I can't break until the draft is done, or I lose the thread of the story.