I've always found that as long as I work intensely, I don't get sick. Colds, etc., happen after deadlines. And that can make a person delay in letting up.
But pressing on and on is ultimately counterproductive, personally and professionally. Right now I'm finding this going-slack business really nice.
Religion and politics blogger Doug Muder has a wonderful post on this subject called The Stages of Rest.
Note: when I glanced back at the title of this post just now, I read it as Bodily Unwinding. Of course, it's more than that.