[m00x]

It's also good to follow the 20/20/20 rule. Each 20 minutes, take a 20 second break and look at something at least 20 feet away (20m works too, for metric users).

Having your eye focus at a certain distance for a long time has an effect on your ocular muscles.

[auganov]

Yes. But ideally this should happen instinctively. Say, if you're thinking, it's pretty natural to defocus or look at wherever. Whenever you're not actually reading or looking at something on the screen your eyes should wander off even if it's just for a few seconds.