Based on what my first pulmonologist told me, everyone's oxygen level drops while they are sleeping. So I wouldn't be too worried about it dropping while sleeping. You may want to discuss with yours the level that it is dropping to though. When I was first put on oxygen, it was only for when I was sleeping, go anywhere and going upstairs. I was able to go around the house (if I remained on one floor without) at first without anything.

There are special guidelines for people with known respiratory issues in regards to flight. The combination of a pressurised compartment and lower levels of oxygen on planes, around 5 percent lower than at sea level outdoors, can lead to a variety of problems for us. Because of the reduced oxygen levels on the plane it wouldn't have been too strange that your blood oxygen levels were reduced but probably not to the degree which you described. I have extrinsic respiratory failure, involving failure of the respiratory muscles, but I think that the majority of people experience intrinsic failure because of some kind of lung disease. From my own experience lung diseases are fairly easy for doctors to identify but extrinsic problems aren't.