When we rest from practicing is when a lot of the processing of what we are learning goes on.
Be sure to rest about ten minutes for every fifty minutes that you practice. You don't need to have been physically playing for those fifty minutes to require a small break, good practice is mentally taxing.
Also make sure that you set clear easy attainable goals for each practice session so that you know when to stop and go on to something else. Otherwise, you'll realize that you just spent two hours playing the same two bars while neglecting everything else. That kind of perfectionism has its place, just make sure you do it because you plan it and not because it happened to be that way.
Clear your mind when you take a break. Drink some water, walk around, stretch out, have a chat with a friend, have a light snack or smoke a cigarette if you like that sort of thing (I don't smoke, but your lungs are your problem). Apparently one of the best things you can do during your break is play with your pets.
And don't turn on the TV, because you are not going back to the piano if you do.