Only until your brain reaches a threshold where it says "OOOOK, we definitely need to start breathing now. Time to wake up". When you reach that threshold, you

Only until your brain reaches a threshold where it says "OOOOK, we definitely need to start breathing now. Time to wake up". When you reach that threshold, you wake up on a very low, subconscious level that disrupts your sleep pattern, which in combination with the lower level of oxygen, is what causes the damage to your system. The difference with a malfunctioning machine is that you still cant get air even after that threshold has been hit (or at least it's very difficult to do so). At that point, your brain kicks you totally out of a sleep state and you awake completely.