If you’re spending a lot of time looking at a screen, you’ll probably want to turn down the blues, to give your eyes a chance: http://jonls.dk/redshift/.
To install, just use your operating system’s package manager (apt, pkg, pacman, etc.) to install redshift. On Ubuntu and Debian, this would be:
apt-get install redshift
Try a few of the following commands, and see which you like better (just run these in a terminal, and kill one before trying the other. It’ll take a few seconds to actually shift the colors on your screen; be patient):