> "I think most of my discomfort from long flights is due to altitude sickness." Seems unlikely. Actual altitude sickness is generally considered to occur only

> "I think most of my discomfort from long flights is due to altitude sickness." Seems unlikely. Actual altitude sickness is generally considered to occur only above 8000 ft, and above 10000 ft is where the real risk begins. Symptoms generally don't appear until 12-24 hours after climbing to altitude. Also, risk of altitude sickness increases with physical exertion, and you're probably not doing much of that on a plane!