Cognitive fatigue is no joke. For instance, in general, the brains of people with dyslexia use 5Xs more energy when reading and doing tasks that involve the dyslexia. It's not unusual for the dyslexic to need sleep after reading etc. Other parts of their brain have compensated and picked up the slack, and that significant additional energy use literally exhausts them. I would assume it is similar with the blind, deaf, etc. Anytime the brain has to move work to another area, or areas, of the brain. The brain is a machine and can demand a lot of energy to do its work.