I’ve got a character who’s asleep in controlled conditions, while the oxygen levels in the surrounding breathable air are decreasing. The character in question is a light sleeper, pretty heavily stressed both physically and mentally, but has enough of a background in aviation and similar disciplines to recognize both his own symptoms indicating hypoxia and what this indicates for his surroundings, i.e., nothing good -- but all this is predicated on him being awake and alert enough to do so. Would the discomfort/unusual sensations associated with anoxia cause him to wake up at any point? (More "is this possible" than "is this a certainty" -- in particular ones like air hunger, less so sleepiness/yawning.) If so, at what point is this likeliest to happen, as far as what percentage of the original baseline level of oxygen is left? At what point is he basically past the point of no return, if nothing else wakes him either? Is this change of state more likely to wake him if it’s a sudden change rather than a gradual one?
Terms Googled: a couple different ones relating to hypoxia (especially altitude hypoxia, which seems like the closest analogue to the gradual draining of breathable oxygen from an environment, but I might be wrong), identification and treatment of altitude hypoxia, air hunger, etc.
Apologies if any of the medical descriptions here are horribly off!