How does staring at a computer screen all day hurt your eyes, etc?

Despite the legends, sitting in front of a TV or a computer screen all day will not cause any permanent damage to your eyes. Your blink rate will likely decrease, causing your eyes to dry out an irritated, but blink a few times and this is fixed. Your eyes have to work a little harder to look at something close (the technical names are accommodation and convergence), which could give you a headache, but this will not cause any permanent problems. From a physics standpoint, a monitor and a TV screen are the same thing. TV screens are made to show motion with less ghosting, but that isn't going to affect your eyes. You generally sit further from a TV and don't look at the details as intently as you do with a monitor. As you read this text, your eyes are working to accommodate (focus on something close) and converge (bring the images from your two eyes into one image in your brain). You probably aren't looking away from the screen very often, so your eyes and brain are working very hard. This can lead to a headache, but not permanent damage. If you watch TV, it is further away and you probably glance around the room more often, so your eyes and brain don't work as hard. Due to your eyes growing (just like the rest of you), people who need glasses need stronger prescriptions over time, and many people incorrectly blame this on watching TV or reading or whatever. Nearsightedness is caused by the shape of the lens of your eye and the size of your eyeball. Nothing you do (other than grow) will affect either one, but people have all kinds of explanations for why their eyes are "bad". One exception - if you have a traumatic eye injury, that will hurt your vision, so try not to get hit in the eye. But it will not cause nearsightedness.