Our eyes work like cameras. Before autofocus cameras we manually focused a camera by moving lenses closer and further apart by twisting the camera’s lens system. Moving the lenses would change the focal length and create clarity.
Our eyes work very much the same way with a flexible lens in the eye changing shape to create clarity. Our eyes are not designed to hold static close focus. They are designed to focus dynamically and continually move. Keeping the same high point of focus (e.g. smart phone) fatigues the system and it becomes sore.
If you do.. or do not wear glasses at all, there are options to relax your eyes. Essilor EyeZen, Zeiss Digital and Nikon Relaxsee are examples of antifatigue lenses. These lenses help reduce eye strain by assisting your eye’s focusing system.