The military goes on O2 at 10,000.

I personally see serious cognitive degradation around 14,000. But I ski and hike in the mountains so I'm used to physical activity up to 10 or 12.

If you are inactive, overweight, and/or a smoker, mild hypoxia at 7-9K might not be unusual. My wife falls asleep much over 8.

Cardio exercise should help, and also check to see if you have an iron deficiency.