It's true and i have a rather valid theory on why this is.
When we're moving, doing stuff, etc, our body notices that we need energy and as such, we hunger for..well...stuff. Vampires need not apply.
When we sit on th computer, we mainly use our brain. Hell, fingers clicking stuff hardly is "energy wasting".
And as such, our body goes into a "sleep like" state with the bare minimum to keep us sitting upright and moving the mouse/tapping the keyboard.
When we re-initialise our motions and start using our muscles, the body waes up, and akin to waking up from sleep, it lets you know that you need hydration/sustanance.
imagine how busy it gets if your thinking of the next big FH thread!