I once was in a Cessna 206 on a flight plan that took us up to 17,000 ft. You can fly that altitude fine, as long as you have supplemental oxygen (via a cannula

I once was in a Cessna 206 on a flight plan that took us up to 17,000 ft. You can fly that altitude fine, as long as you have supplemental oxygen (via a cannula in our case). We took off at about 3AM, and I fell asleep immediately. Woke up somewhere well above 10K, dizzy as hell and realized what I'd done. I put in the cannula and the dizziness left within a few minutes iirc.