I don't mean it's literally bad for your eyes - there may well be benefits to constantly shifting your gaze/focus/etc. I can't comment on that.
I'm talking about "natural" eye movement being a well-known design principle. This means that eyes very naturally follow a Z pattern that doesn't exceed the periphery of your focus. So if you load up a fresh page, your eyes will naturally look top left -> top right -> bottom left -> bottom right, bounded by your standard viewbox (which is about 800px-1200px wide at 96dpi and 1-2ft, the standard distance most people sit from their monitor). So you usually stick your most important information along that pattern. Anything outside this boundary requires an extra "look", which means a slight hesitation/delay on the part of the user. That's not to say the space is unusable, just that you put your most common/important features/information along this flow.
It's also not an ironclad rule, but it does work very well. When I went to the old GitHub, my eyes always followed the same pattern: repository name (top-left, to make sure I was on the right page), account (verifying that I'm logged in), branch, clone/download, then file list or more commonly, the README (bottom-left). It was a very quick, natural way to navigate a random GitHub page.
Now, I literally have to move my head to do this. The weighting also feels completely unbalanced, like there's too much information on the left it's all slanting in one direction.