In a typical design of a nuclear reactor, control rods are suspended above the core by electromagnets such that, in the event of a power failure, they automatically fall under gravity into a safe configuration.
A similar principle applies to my workspace, which by design has no chair:
When I get tired, I'm compelled to leave the screen. If physically tired, I'll set a timer and take a nap; if just mentally tired, I'll go on a walk.
In either case I'm prevented from accidentally droning away.