For me the mental fatigue is mostly related to context switching, being disturbed (slacks, meetings etc but also side quests to the original problem), and working on a fuzzy problem.
Working a few hours in such environment is very fatiguing.
On the other hand, when I work on a single thing, no disturbance, clear problem definition, having all necessary skills to do the thing, I can work 10h and it's not fatiguing.
Working a few hours in such environment is very fatiguing.
On the other hand, when I work on a single thing, no disturbance, clear problem definition, having all necessary skills to do the thing, I can work 10h and it's not fatiguing.