During the day, when your office or other room is lit mostly by the sun, the natural light has a blue tint to it. As does your PC's monitor. Which is all fine. Your monitor is comparatively bright, and is visible comfortably in daylight.
But as evening falls, and you turn on the electric lights, your room takes on a less blue and more of a yellow hue. And the overall level of light is less than during the day. And yet your PC monitor is still at full brightness, and still blue, which makes your eyes tired.
Which is where a brilliant program called Flux comes in. I've written about it before, but there have been numerous updates so it's about time I mentioned it again. If you don't have it, and you use your PC both during the day and at night, you need it.
Flux automatically and almost unnoticeably adjusts your display so that, as the sun sets, your screen loses some of its blue tint. It makes your eyes more comfortable, regardless of when you use your computer. Flux knows your location (though you can set it manually if required), and therefore needs very little configuration.