@ tdracer, that is very interesting. So more than likely, it is the constant low-level stress for hours on end, combined with moments of high workload and stress, that leads to tiredness and fatigue.
Obviously pilots are under this pressure all the time while in control of an aircraft.
Makes it all the more important to de-stress. Physical exercise is a very good way of de-stressing, and keeping fit, slim and healthy gives the body the best chance of coping.