Believe it or not, eye fatigue is something that can be worked on. I used to practice focusing hard on a very small point on the wall for as long as I could, till my eyes couldn't hold the focus, then I would relax them for a short time and do it again. After working up to it, I then began to be able to intensely focus on the table for an entire long match without feeling eye drift. And the added benefit.. While you are able to focus hard, you tend to drop into dead stroke.
The eye muscles are an important part of being able to play top level pool for longer periods of time, and should be exercised... For me, specifically, I just found out that my eyes are pretty much equal strength, and that my "dominant" eye can flip between both eyes depending on the distance/angle, and do not consistently do so. This is BAD for maintaining the correct aim line. I think I overcame this as a young man by practicing focus exercises that forced me to maintain the focus out of the same eye throughout a playing session, which kept my line of aim consistent.