To everyone mentioning redshift/flux; they are great (I use flux on every computer I have) but they do not solve the issue being raised here. Bias lighting keeps the light levels behind your screen (or whatever you are looking at) higher, so that your irises don't have to contract and expand nearly as often or as much. This leads to less eye strain. (http://www.cinemaquestinc.com/ideal_lume.htm seems like a good explanation, first hit on google)
Redshift/flux vary the color temperature of your monitors. This keeps your internal biological clock from thinking the sun is up at midnight because of a bright white high-color-temp monitor. (More info at http://stereopsis.com/flux/)
Actually if the light is behind the screen, your iris will adapt to different levels every time you look towards any other direction than the screen. The best solution is to have constant level of ambient light all around you.
I meant to upvote, and fat-fingered downvoted you, and greyed your post. Hope someone can correct my mistake (I fixed your karma on another post). Thanks for the clarification.
Redshift/flux vary the color temperature of your monitors. This keeps your internal biological clock from thinking the sun is up at midnight because of a bright white high-color-temp monitor. (More info at http://stereopsis.com/flux/)