it might be pop sci, but the explanation I heard long time ago is: creativity is boosted by beta brain waves, (associated with daydreaming) Physical activities

it might be pop sci, but the explanation I heard long time ago is: creativity is boosted by beta brain waves, (associated with daydreaming) Physical activities that don't fully engage your brain (walking, exercising, taking a shower, etc..) increment beta waves. On the other hand, increased focus and alertness, is associated with alpha waves, coffee produces that effect thus reduces your creativity