I have at times spent prolonged periods writing software and did end up with eyes so sore that I could not look at the screen. The company Nurse's advice was that I should only spend fifty minutes in the hour at my pc, then I should get up and walk away from it for ten minutes. I should spend that time focusing on things at varying distances, all a lot further away than my screen. There appears to have been no lasting damage in my case, but it seems I was lucky. Hope this helps.