And then there is the added risk of oxygen shortage, which i presume is a sliding scale.....
Meaning: Are we more likely to become drowsy with sleep at 9000 ft than at 2500 ft?
As we fly further we tend to fly a little higher (it's more worthwhile climbing on longer trips?), perhaps also because we undertake longer trips in better weather, so climbing is more likely in CAVOK weather?
So everything conspires to generate drowsiness....