This is very strange to me. When people are on oxygen here, in addition the regular machine, they are issued tanks that do not require electricity that will last at least 24 hrs. in case of emergency.
It may be that some people who are asleep don't wake in time when they are suffering from a lack of oxygen. That is, for example, why carbon monoxide poisoning is so dangerous. If you are sleeping your body doesn't notice the initial symptoms.
That’s how my mother (RIP) had it. Backup oxygen tank.