let's face it, mental work is truly exertion, and in fact can be harder than any physical work. Not only are you mentally going through the wringer in session, but your body knows the tension and stress, too. And after you leave, even (or maybe especially) while you are sleeping, your brain is processing away, it doesn't sleep really. It's no wonder you are tired.

Hmmm
After finishing this paragraph I had to go back and change what I actually typed, which was, "not only are you mentally going through the writer in session"... now what's that all about....