But its worst effect may be causing you to change your posture to an uncomfortable one, in order to see well. The best solution is to find a way to darken the area around the screen. This problem occurs mainly on screens with light letters on a black background. It depends on your age, the quality of the print you're reading, and other factors. There should be plenty of light for easy reading, but too much can, depending on the person, cause eyestrain.
Eyes are strained more by close viewing than by distant viewing.
If you gaze at something too long, your eyes can tire. Eyes need to focus at different distances from time to time. If two objects are only a couple of inches different in their distance from the eyes, the eyes actually do NOT have to refocus to look from one to another. In general, keep viewed objects at about the same distance if you have to look back and forth a lot. Rarely, according to optometrists.