If feels good to rest after you have worked hard on something and it is a good thing to do, too. Sometimes you need to rest because you have been doing physical things and sometimes you need to rest because you have been doing hard mental things, like homework or trying to understand something that isn’t easy. Your body doesn’t mind too much when you do mental things but you do have to remember that your body really needs to move: it’s designed to move, to work, to bounce around and have fun. So when you go walking, or play a game outside, the body knows it’s doing what it was meant to do, and it’s happy.
However, when you are doing something mental like reading, writing, sitting and watching a movie, texting or playing a computer game, your mind might be getting a lot of movement, a lot of exercise but from the point of view of your body, you’re just staying still, sometimes for hours. If you have ever tried to stay still, very still, on purpose, you know how hard that really is: your body keeps wanting to move. Your hand jiggles or your foot twitches or at least your body feels tired.
Tired? How can you be tired form not moving? By forcing your body to be still too long, you are actually making it work very hard indeed but it isn’t the kind of hard work that gives your body benefit. When you are too still for too long, you forget to breathe deeply, and your lungs aren’t giving you enough air. When you sit too long however much fun your mind is having, the blood in your veins is dragged down by gravity and makes little pools here and there, where you’re sitting or lying. Those pools of blood are hard for the body to move.
So: move around, wriggle your feet, stretch your arms and move your head even when you are working with your mind. Your body will love you for it.