You need to work on a tough problem or study a topic that requires your full mental attention and concentration. Which of the following are you most likely to do:
1. Work without a break until it is done. You only take a break if forced to.
2. Work without a break until some fixed time limit, take a short break and then keep going. You *plan* on taking breaks even if you don't feel you need them.
3. Work without a break until you feel like you need one. Then you take a break.
4. Other?