When you have trouble staying focused during your workday, activities at home or your studies that could be a sign that you need an exercise break. This does not mean you need to pack up and head to the gym! For your exercise break, I am suggesting that you take a brisk walk, run in place or do some jumping jacks. Just do something simple to give your self a break and get your blood pumping.
Spark: The Revolutionary New Science of Exercise and the Brain by John J. Ratey, M.D. is a good resource if you are interested in learning more on this topic.