I think I can sympathize. I tend to be a homebody, a while back someone I worked for actually made that comment, also that even at work (at my former employer) I tended to hole up in my cubicle and not leave for the rest of the day.

I have a theory (oh no, another one of Kenjhee's theories!) that this is part of an immune system response. How so?...well, what does a sick animal need most of all? Rest, so it can recover from its illness. Behavior is something that can be hard-wired into the genes, just like any physical expression. Something about a brain injury triggers this inappropriate response (or maybe, partially inappropriate, as we TBI survivors are sick, after all).