Good question smiley41.
I love aviation and I am a passionate flyer, but! .. in every climb and descent I yawn... feels like there is no oxygen... and its not only me who yawn, I have spoken to other flyers and they report the same. I even have seen it with my own eyes. So there is something going on.. I believe it has something to do with the pressure changes not as much with the oxy.
Cheerio!