When you're mentally exhausted from writing, is *all* mental effort really worn? I ask sincerely, out of curiosity, because I would have figured that different aspects of mental effort would still be going strong; the math centre, for instance, or visual-spatial, or strategic, or even other forms of pattern recognition.
At least, that's how it's worked for me; if I can isolate the aspects related to writing out, I can find things to do.
I'm with vito_excalibur on the extra weight/tiredness. WTF??