You read a big serious book, full of intense heady content, concentrating is key. You finish the book/novel/manifesto, and then give your mind a rest with an easy "beach read".
Painting is like that too sometimes. I do these studies that are so involved that I am completely immersed, and hardly take time to breath. I'll work on something for days, hours that feel endless and fleeting at the same time (that's how you know that you're in "the zone"). When those paintings have been completed, I find myself somewhat mentally drained , and n need of recharging my creative mind.
Enter the simple painting, something lighter, brighter, completely spontaneous and simple. I might use these as a way to experiment with new materials and ideas. After all, there's no pressure to make a masterpiece, and usually, that's when it happens , a breakthrough style or technique. It pays to give yourself some slack sometimes, and the result may surprise you.