In my experience, it's not only for the times you look to the side of the screen. Working in a dark environment with a relatively bright monitor is very harsh o

In my experience, it's not only for the times you look to the side of the screen. Working in a dark environment with a relatively bright monitor is very harsh on the eyes, even you stare at the monitor all the time. So reducing the contrast between the ambient and the monitor is key to reducing eye strain, at least for my eyes. You can even improve this, by choosing a dark green or blue background, and some beige like pale color for foreground in you IDE or text editor.