Hippus is the normal involuntary small rhythmic pulses (dilation/constriction) of the pupil. The frequency and size of hippus movements are influenced by light. There is a school of thought that backlit screens cause more fatigue and strain because they induce more intense hippus movements. This would be why many people enjoy reading more on paper or e-readers vs. tablets/laptops/phones. There is not much research on this topic to make recommendations.