Need For Cognition

In psychology, "need for cognition," or NFC, is a personality metric that gauges how much an individual requires thinking-heavy activities to be fully engaged and fulfilled (and to some degree, how much they enjoy such activities).

"Thinking," in this context, generally implies the expenditure of cognitive effort toward better understanding and organization of reality, though this definition is a bit fluffy and varies substantially between those doing research and working in this space.

That said, if you're feeling exhausted from studying an unfamiliar language, working on a difficult crossword puzzle, or exploring a concept you're keen to understand, it's likely that you're engaged in the sort of hefty cognitive endeavor to which this metric refers.