Back when they started making the international flights nonsmoking even the stewardesses were complaining about the lack of fresh air. Of coarse the airlines loved it because it saved a little fuel. Someone told me that the cabin pressure (and supposedly the O2 level) is equavlelent to 10,000 ft. elevation. I used to fly 150,000 miles a year and on virtually every flight I went to sleep shortly after takeoff and then woke up about the time we got to cruising altitude. Guess that was my body adjusting to the lower pressure/O2 content.