I know this is an old post, and the consensus is that the Garmin measurements are not medically accurate… but I still want to add I have a Garmin forerunner 955, I live at 1500 foot altitude, and I’m normally about 95% nightly average, when I go to Colorado and stay at 10,000 foot altitude it drops to 71% nightly average for me. After a few days of acclimating I get the mid 80s.