Physical fatigue is lack of ATP and happens through the day as oxidation builds up. Cognitive fatigue is a build-up of used neurotransmitters that also build up over the day. These two processes interact with each other though where neurotransmitter reuptake uses a lot of ATP. I see that connection between them as ego depletion. If true, the best solution to it is a nap because that will help clear the junk out.