If your eyes are like mine and got older, they're probably not focusing anymore on the same plane.. my left eye and my right eye without glasses focus on a very narrow plane. In the past there would be overlap with room to move back and forth.
You can see the focal distances by covering one eye and moving the book in and out to see where it is sharp focus and blurry, and then do the same for the other. If you're holding it outside the optimum zone, you're going to experience tiredness unfortunately.