Digital eye strain shows up as tired, dry or blurry eyes and headaches after long periods on a computer, tablet or phone. It is uncomfortable, but it does not cause permanent damage to the eyes.
Try the 20-20-20 habit: every 20 minutes, look at something about six metres away for 20 seconds. Blink fully, because we blink less while concentrating. Keep the screen about an arm's length away and slightly below eye level, make the text larger and reduce glare from windows and overhead lights.
An out-of-date prescription adds to the strain. From around 40, spectacles made for screen distance can also make a real difference.
For children, balance screen time with outdoor play, which also helps protect against short-sightedness. Our article on slowing myopia in children has more.
There is no good evidence that blue light from screens harms the eye. Dimming screens in the evening and putting them away before bed is good for sleep.