So many of us at grad school/seminary come across this, whether we realize it or not. We work so hard, so diligently with every fiber of our being. We try to keep up with our reading assignments ... write papers ... attend chapel and church services ... work in our churches ... take care of our families until ... We can do no more. We have Hit the Wall.
Pace yourself. It is never any fun to hit the wall. Ask those two guys above. ;)