Since there are few comments let me start off with perhaps an unusual anecdote. I have seen hypoxia (as in insufficient oxygen) with people who do the Wim Hof

Since there are few comments let me start off with perhaps an unusual anecdote. I have seen hypoxia (as in insufficient oxygen) with people who do the Wim Hof Method (a breathing technique to keep you from freezing to death). It’s very disconcerting to see as the people practicing it black out and are irresponsive. Luckily, there was doctor supervision and they rushed with oxygen masks to the persons with insufficient oxygen whenever it happened as this was a medical experiment. It took about 2 minutes for some to be conscious again and respond to: what’s your name? My own experience was a blackout for 10 seconds and I was watching at a stopwatch at the time and I remember seeing the 10 immediately change to a 20, followed with the question: what’s your name Melvin? Apparently they asked me that from 11 to 20.