I admire the people who can stare at a computer monitor for hours on end without a break. They seem to have the superhuman ability to not wear out their corneas or get a splitting headache. I may have had that ability when 1 was younger, but it's certainly long gone now.
It doesn't hurt to take a step back from the computer every now and then. Getting up to stretch, walk around, play some video games, or whatever you want to do is a good way to recharge your batteries.
At the very least it's a good way to come back to a particular panel that's frustrating to work on. Sometimes walking away from the point of irritation for a while, and then coming back to it can set off a light bulb in your head, and suddenly you find yourself finishing the problem in no time. It's certainly cheaper than punching your fist through the computer monitor in frustration.
Also, much like finding a comfortable setup to work, taking a break is good for the body that may have been stuck in one position for a while. Do whatever it takes to get the blood flowing in your system: Stretch your muscles, roll your neck around, grab a sandwich, anything. It'll help you both mentally and physically, and that's good for your artwork in the long run.
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