There aren't any definitive guidelines, but it's generally accepted that you should try and take breaks by looking away from the screen every ten to fifteen minutes. Additionally, you should try and move away from your computer every two hours to give your eyes and your body a break.

Computer users tend to blink less, so they can experience dry eyes - this is particularly noticeable if you're working in an air conditioned office. One way to overcome this is to try and remember to blink every time you hit the return key.

Find out more about computer eye strain