During airline flights we are subjected to cabin pressures which are similar to being at 8,000′ of elevation. Stanford researchers observed a striking decrease in blood oxygen (SpO2) levels during airline flights. They subjected a participant to 96 different flights for measurement. The flights ranged from 23 min to 829 min in duration. A drop in SpO2 levels of 96% or lower occurred in all flights and many flights were less than 94% for most of the flight.
Researchers went further to associate fatigue with SpO2 levels by issuing a psychomotor vigilance test and quantified fatigue based on visual response times.
One unexpected finding is that toward the end of a long flight SpO2 levels would begin to rise. The increase is believed to be due to a adaptation or physiological change after rest/inactivity.