Determine the times you are most focused. Study when you are not likely to be distracted. Study is most effective when you have had sufficient rest. Pulling all nighters will not adequately prepare you for taking tests. When you have to take 3 or 4 finals in a row during final exam week, you'll need to be rested to be able to stay focused.
Exams are stressful and drain energy.
Stop studying when your attention begins to wander. Charles II arg.