Set Color Temperature
Tools such as Redshift and f.lux set the color temperature of your screen according to your surroundings based on the time and location. When the sun sets, colors become warmer, while colors become cooler when the sun rises. The warm color temperature at night is supposed to make your eyes hurt less and make you sleep better after working at the computer before going to bed. OpenBSD developer Ted Unangst published a simple UNIX command-line tool called sct (set color temperature) which adjusts the color temperature of your screen according to the temperature parameter that you pass to it. The tool is written in C and is just a couple dozens lines long. Since it does not automatically adjust the screen colors based on your location and time, it is less bloated than Redshift and f.lux. I cleaned up Ted's tool to get it to compile on Ubuntu 14.04 and implemented some minor fixes along the way. My modified version is available from GitHub.