When it comes to writing the most important thing to keep sharp is your ability to think creatively. Creativity is actually a component of the subconscious mind and thus to access it you have to bypass the conscious filters and just let it run wild. You do this by distracting the conscious mind and not depending on it to complete a task- that's the job of the subconscious mind if you let it (and there is no fatigue in using this).
Conscious creativity is like riding your car with the parking break half way up, sure it works but it will wear you out in the process because your conscious mind is consistently filtering it to the point where most of it doesn't get through. Think of it as the difference between listening to an audio book and reading intensely on the screen. Accessing your subconscious mind is a skill set that takes time to develop but the best way to do this naturally is to learn to multitask well.