is aching from too much hard work.
I've been very good and doing my homework, but it's involved a lot of thinking, and I don't really like thinking! Well, not about the stuff I've been thinking about. Hows and whys and wherefores and... you get the gist.
The good news is it's made me very tired, and I might well manage a full nights sleep tonight. Like, more than four hours or so.

Thinking is hard work . . it always tires me out too