Are you computoholic? Yep that's a word I just made up. Not present in any dictionary. Do you tend to get up from the seat in front of your computer at the evening with the sudden revelation that you didn't make it out of the room even two times within the day. Or are you suffering from eye problems due to excessive usage of computers. Then here is some thing that may worth checking out. Workrave is a small software which sits behind your back and says to you when to stop!. Yes you heard me. After certain interval it rings to you by giving visual indications that you should probably take some tea break or if not some exercises for your eye or your back. It plays you animations as well as how to do them. Pretty cool huh? The time intervals are configurable. So it is advisable to the set the break intervals that suites your work style. But don't cheat and set the interval to 4 hrs. You know what I mean. In Ubuntu you can install it by
$sudo apt-get install workrave
It is cross platform. Google workrave you will get many references.