I think you all is gettin' the wrong point. Using a computer IS a sedentary activity; it IS rest. It doesn't Require a different sedentary activity to 'relax', it requires some 'activity' because it's a living, breathing machine. It even animates if you let it. In fact, it needs it. Just try to remain totally still for as long as you can and you'll find it increasingly difficult since your body has it's own mechanisms to get your ass, and every other part of your body, Moving to regain the advantages that accrue from physical movement, such as increasing your blood circulation to the extremeties that aren't well reached by the pressure from the heart muscles alone. You Think you need to Rest because your brain activity is showing signs of fatigue and stress because you're forcing it to use the limited nutrients and oxygen it can get from the limited motion of your sedentary position in a chair for hours.
If your brain doesn't Get proper circulation, you're in trouble. The brain will actually put you to sleep if you don't vary your mental activity. Does that mean you're tired? No. It means that the brain is designed to continually process New information; to form New associations, and to be on alert for danger by taking in all the stimulus around you in the form of auditory and physical sensations. If you've deadened all possible elements of these sensations by sitting in a comfortable chair in a quiet room with little distractions, your brain is just taking in the same stimuli over and over, sort of like staring at the lines in the middle of the road at night, and your brain will purposely put you to sleep.