Devil 49 has posted about the scheduling problem in the past and seems to have done a bit of research into the matter as his views seem very well founded.
Compound the risks of night flying doing EMS and pilot fatique must be considered a contributing factor in many of the crashes.
I have found myself "waking up" on the way back in from a scene...and wondering if I had in fact gone to sleep or just drifted off mentally and thinking I had been sleeping. That usually was about night five of seven.