Sitting in front of screen all day is not healthy. Well, if it were a screen door, that might not be too bad, but we're talking about a computer screen. In fact, just sitting all day isn't healthy, either. Let's face it: Do you think the colonies would have won the American Revolution by lounging and playing World of Warcraft all day?
Here's a simple list of 10 ergonomic upgrades for your workspace. It's pretty simple stuff, like elevating your laptop screen/monitor to eye level, paying attention to your desk spacing and keeping your eyes from straining too much.
Personally, I think it's a good idea to not stare in one place too long. If you're working on a term paper or run into a hellacious bug in your code, get up and walk around at least once an hour. I should also mention that using a cat for a wrist rest is not a good idea. I suppose cats are one of the few living beings who can get a pass for sitting around all day.