Yes, I think so. You have to keep up the momentum of learning and mastering very difficult concepts in physics. Many aspects of physics education involve mastery of skills in problem solving. These kinds of skills are easily forgotten without repeated practice, and I think taking a year off would be a disaster. You would come back to a program rather rusty, and have to compete with fellow graduate students that did not take time off, and for whom the current batch of problems are just a continuation of what they were doing a few months/weeks ago.