Wikipedia
Workrave
Workrave is a free software application intended to prevent computer users from developing or aggravating occupational diseases such as carpal tunnel syndrome, repetitive strain injuries, or myopia.
The software periodically locks the screen while showing an animated character, “Miss Workrave”, walks the user through various stretching exercises, urges them to take a coffee break and sets a daily work time limit after which it automatically triggers an action, such as supend the machine.
The program is cross-platform, and dependent on the GTK+ graphical widget toolkit, and also on other GNOME libraries on Linux. It is also available for Microsoft Windows.