I spent so much time on the computer in the dark over Easter that I gave myself eye strain. If you're noticing that your eyes are feeling strained after using the computer, even for a relatively short amount of time, then don't use it so much. Even though eye strain isn't really permanent damage, it lasts for ages and you'll find it hard to overcome when you're having to revise a lot as your eyes won't have a chance to rest.
Some stuff my optician told me with regards to eye strain; always have a light on in the room when using the computer, take a break every 20 minutes or so to look at something neutral and distant, make sure your chair is set so that your eyes are level with the top of the monitor, and don't use the computer so much.