20 years ago computer monitors were harmful for our eyes because of the radiation. But it's a history! Today we have Displays with no radiation LCD (Liquid crystal display)...
But! They have other problem...not technical. The only problem of LCD is that during all time we look only at one point. 1,2,3, sometimes 5 hours we sit in front of the display, and look at one point. The crystal in our eye does not change it's form. The crystal is some kind of muscle, and like any muscle, it needs to change its form. Every 30-60 minutes it's recommended to take a break, to relax, to relax our eyes, to look somewhere far, to do some exercises with eyes. For the beginning you can see this manual.
But some people are so addicted to computers (like me) that they often (if not always) forget about the break...
The first mail system which tried to fix this problem was GMail. They have an option that disables your mail system for 5 minutes. So time after time you can disable your mail.
There are also a few offline applications for the whole computer. They disable your screen for a few minutes every 30 (and more) minutes. The one I have just found is Eyes Relax
You can customize the options of duration, period of break, add some music to the break time. And lot's of fun!
So take care of your eyes :-)
Special Thanks to Aysel Mammadli