I've climbed in the Andes in the past, getting as high as 6700m. I would really guess that poor sleep is the major factor aside from oxygen deprivation. It's ve

I've climbed in the Andes in the past, getting as high as 6700m. I would really guess that poor sleep is the major factor aside from oxygen deprivation. It's very difficult to sleep at those altitudes, both from lack of oxygen and the difficult physical conditions. The human body's breathing is controlled by CO2 in the blood. If you aren't getting a lot of oxygen, you aren't making enough CO2, so you are not breathing enough. This is the main cause behind altitude sickness. It takes time for your body to adjust its breathing rate. This issue is worse at night. This is the reason behind an adage "climb high, sleep low." Many people even get Cheyne-Stokes breathing, which causes one to wake up hyperventilating kind of like sleep apnea.