Visual stress is induced if--while watching your screen--your peripheral vision is exposed to a light intensity substantially brighter than the brightest regions of your display. The color science term surround refers to the area perceived by your peripheral vision while you are looking at a display. In addition to disturbing your peripheral vision, a bright surround increases your ambient illumination. Try to establish a visual surround that is darker than the brightest white of your screen.
It is beneficial to have a visual reference to the outside world--such as a window to the outdoors--while working at your computer. If you have a window, make sure you sit so that the window is far enough to your side that it does not impinge your peripheral vision, but not so far behind to reflect in your screen.