I have read many scientific studies on athletic performance at altitude. Active high altitude residents are always looking for ways to improve. As we age we experience a loss of speed and endurance, even with regular training. Some of this is inevitable, but how can we know if there is something else affecting our fitness?
I started sleeping on oxygen 9 months ago because of high blood pressure, which was instantly cured. Now I find that my strength and endurance have improved during the last few months. For example, I was rowing 13400 meters per hour with several brief pauses last fall, and now I am at an all-time high of 14100 m per hour with one pause. My running feels better, I’m back up to 6 miles from 4.
There are other factors that could influence this. In 2012-2013 I was on 17 pills including prednisone and had four surgeries for tongue cancer and myasthenia gravis. I was able to continue working out daily although part of that was less intense, such as yoga. I also had rotator cuff surgery. So my current fitness improvement could just be a rebound from overcoming those health conditions.
The only way to know for sure is to do a randomized controlled double blind study of athletes performance on and off nightly oxygen, or study the same athlete with and without oxygen. This is not an immediate effect, so months or years of observation and measurements would be needed.
In the meantime, if you live above 2500 meters/9000 feet and are losing stamina or strength consider having a night time pulse oximetry test to check for hypoxia during sleep.