Contrast is what lets you visually determine an edge. So it is vital in recognizing shapes and reading. Too little contrast in the foreground and you will suffer eye strain when trying to focus too hard, like reading in low light. Too much contrast in the foreground and you will suffer from eye strain because of the harshness and glare.
Color has a lot to do with relative brightness as well. A bright color is bright. A contrasting color is high in contrast. Bright, contrasting colors can cause more eye strain than black on white.
Dark figures on light backgrounds are easier on the eyes than light figures on dark backgrounds. Using a dark gray (80-90% black) and off white is easier on your eyes, yet has enough contrast for easy and efficient object identification.
You want the brightness of your background to be the same or slightly darker than the foreground. Your peripheral vision picks up a lot of what surrounds your work site. If it is bright, colorful, or busy it will strain your eyes. Move the photos and nick-knacks to another area and keep your background a matte gray or tan.
If using a screen in the foreground ensure the contrast is set appropriately for its use and ambient lighting conditions.
