system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: why do humans form blood clots on 8 hour flights, but not during 8 hours of sleep?
gpt
Lower cabin pressures mean the partial pressure of oxygen you inhale is lower. So your blood is less oxygenated and that increases clotting.

Also you don't move enough on a plane (people move quite a lot while asleep in bed). So your veins are kinked at joints, slowing flow, and you don't use your large muscles very much (muscle contractions aid in the return of blood through the venous system).