For me , the main cause of fatigue is boredom. There's so little to do, and so much time to do it, that I find it difficul to motivate myself after ten hours staring out the window. There's only so much you can talk about with someone that you'd probably never see outside the job, and the papers only take an hour or so to read. Bring on in-flight entertainment for flight crews.....though I've seen some interesting stuff on the cameras...