Whoa. I think I know how you feel. When I become mentally fatigued and can't seem to concentrate, my thoughts (though seemingly meaningful) drift to obscurity and my vision changes its mode of focus. by this i mean instead of concentrating on a specific point, I seem to see more evenly across my field of vision. It seems to be associated with daydream. Not so abnormal it may seem, but as mentioned, not good while driving. When I catch myself and try to bring back my focus it takes much mental effort. The result being i get very tired. This whole thing seems to coincide like i said with mental fatigue or too much stimulation such as loud environments, crowds or driving at highway speed.

Any thoughts?