Yeah, yeah, we know, we can hear you now: "Holy smokes! You mean that sitting nearly motionless for hours on end isn't very good for me?! Be sure to let me know when you work out the whole 'water wet, sky blue' thing, assholes." It's more complicated than that. So quit it with the sass.
There are these neat little communiques between your stomach and your brain called "orosensory signals of satiation." They're the signals your stomach sends off when it's full to let the brain know when it's time to stop shovelling Cheetos down.
Unfortunately, orosensory signals are like anemic little schoolgirls when pitted against all the other things that constantly vie for your brain's attention, and have a nasty habit of getting shoved aside by bigger, stronger signals. Distracting signals like these clog up your brain all the time when you're doing something fun, like surfing the net.
So you're sitting there, eating chips with one hand and moving the mouse with the other. Have you had four handfuls, or five? How full was the bag when you started? Don't ask your stomach, it's retarded. And your brain is too busy handling the sensory information provided by that .gif of the dramatic hamster to care.