Your brain is like a muscle that tires as muscles do. If you use it more it will improve, but use it for too long in one day and it will tire. This is normal. Unless you give your brain a break it will only get slower, and every quote that you type with a lower accuracy will train your brain to do the wrong thing. If you do not stop, make sure you have the patience, grasshopper, to let your WPM go down to increase your accuracy so you can do better next week. I'm no doctor, but you can trust me.