Lighting & glare
The fix
Put the brightest light beside you, not behind or in front, and match screen brightness to the room.
Why it happens
Glare and a screen much brighter or darker than its surroundings force the pupil and focusing muscles to keep re-adjusting — extra ocular work you never notice you're doing.
If you ignore it
Poor lighting quietly amplifies every other strain signal, turning a manageable screen day into a headache-prone one.
Wellness & ergonomic guidance, not medical advice.