I was listening to a local radio guy talking about this today. He used to be out of Sacramento and covered the Raiders when Gruden was coaching there; said that on Tuesdays every week for the press conference, it always looked like Gruden hadn't slept since the Sunday game. People there estimated that he regularly spent 15 hours a day in the team facility, plus living an hour away, plus eating at some point, so figures he got maybe 4 hours of sleep a night on average.
Pro coaches are under an incredible amount of stress, and with being on a "hot seat" like Kubiak has been this season, it's not so surprising to see it finally culminate in a serious health issue. There have been proposals before to limit NFL facilities to 7 AM-7 PM hours - may not be a bad idea for health reasons for staff as well as players.