While mental fatigue makes realistic sense, as said, for a playable game, I'm not sure it adds anything. If I have to rest my character for 10 minutes every hour, that is just annoying and doesn't really add much. I just put my character in a safe place and go off and do something else. Or it means I sit my character around for a while before I save or whatever else. The idea of physical fatigue is being discussed as a balancing issue (eg, shouldn't be able to fly or swim all day). I think there is some need to balance that, and also apply same rule to other physical activities in the game. I don't recall there being any issue of balance with characters being able to stay awake all the time, and thus, I don't see mental fatigue as something that is really needing fixing right now.