Low O2 and HIGH CO2 make you fall asleep (and there is no "happy ending" because high CO2 causes unpleasant and horrible psychosis, still looking for the link to the research).
For the uninitiated:
CO2: The rebreather incident
PCO2: The Dark Matter of Rebreather Diving | DIVER magazine
DO WATCH THIS: http://www.hse.gov.uk/diving/video/hse_diving_shell.swf
The crazy thing is that all these rebreather gurus could not find a solution, but I did in my garage long ago.
In any event, that is not enough.
Poor equipment and poor systems, procedures, and control inevitably lead to fatalities. The whole thing is f-up at the moment as exemplified by this fatality which demonstrates a whole chain of errors from design and manufacturing, to certification, to training, and to use and prior product delivery.
It don't get more f-up than this (take it from the guy who has two high power rifles recalled in the U.S., but not in the U.K.).