Overworked Eyes, article.
In an effort to reduce the amount of energy consumed, screen manufacturers use very narrow bands of the visual spectrum to light the screen. Combined with overhead fluorescent office light, the harsh artificial light is far from full spectrum daylight that the eye likes to see.
[...] Dr. Anshel suggests, "Make sure you have your computer far enough away to comfortably see it. Ideally, the center of the screen is 7-10 inches below your horizontal line of sight. Move your lights to where they don't cause any glare and to where you don't have any bright objects, like a window, in your field of view when viewing the screen." He also suggests a series of breaks during the workday. The smallest should occur every 10 minutes and can be as short as 10 or 20 seconds. To properly relax the eye muscles, you must look at least 20 feet away. At least once each hour, take a bigger break that includes standing and stretching.
I have found that reducing contrast helps. Computer-, TV-, and tablet-screens should not be much, much brighter than the room. So turn down the brightness of the screens (since a while now I have my big computer screens at about 50% setting), and adjust the room light so there's not a big gap in brightness.
Also, like they say, variation is important. Don't glare at the same screen at the same distance for hours at a stretch. Break it up with breaks or with other things you need to do, where you use your eyes differently.