It turns out monitor brightness isn't the only thing to blame when this happens. Color temperature, measured in units called kelvin, has a lot to do with it. The simplest way to explain color temperature is in terms of tint: You know how the color white sometimes seems "warm" (a bit reddish) and sometimes "cool" (bluish)? That's color temperature at work.

With their typical cool tint, computer monitors look great during daytime hours. But once the sun sets, monitor screens look much better if their color temperature is adjusted accordingly. And that's what f.lux does.

F.lux takes your location and the current date into account, and can tell when the sun is going to set every day. Right around that time, with F.lux running in the background, you will notice a "virtual sunset" happening on your monitor. Everything seems to mellow down a little bit, every shade of color growing slightly warmer and cozier than it was during the day.

You can set the adjustment to happen very gradually over the course of a full hour, or quickly (in twenty seconds). When using the fast adjustment option, things visually slow down while the adjustment is being done, but it only takes a moment. Adjusted, the monitor looks completely natural, and after a while you may even forget the color temperature was ever changed.

If you're doing color-sensitive work, you may want to disable f.lux temporarily. Unfortunately, you can disable it for only one hour--no more, no less. If you need to disable it for more than an hour, you simply have to quit the app (and then may forget to switch it back on--that's what often happens to me, at least).