This attention to the screen seems to go on for a long period. Your eyes need exercise. Look away from the monitor to focus on different distances in the room. At various timed intervals, objects appear, leading the eyes around the screen. Shoulder, arm, and hand stretches .
Outside light competes with the monitor screen.
If the monitor faces a window, shut the blinds or move the monitor away from the windows or other sources of bright light. Reflect light away from your eyes. Glare is a problem when light bouncing off the monitor strikes the eyes. If possible, tilt the monitor if it is reflecting overhead light. In schools, computer labs require less area illumination than a typical classroom.