Writers do an incredible amount of sitting. It's a solitary job and our brains get quite the workout, but it's good to take a break once in a while and move about.
Go outside and get some fresh air. Move those joints. You may still be plotting a scene in your head, but you'll be pumping blood to your brain. A change of scenery, a little sunshine and deep breathing will help your creativity. Give yourself permission to stop working once in a while and live a little.