F.lux is a nice applet for Ubuntu. It adjusts the color temperature of your monitor based on the time of day. It makes your monitor “sunny” in the morning and imitates an indoor lights environemt during the night.
To install F.Lux on Ubuntu 12.04, Ubuntu 12.10, Linux Mint 13 or Linux Mint 14, do this:
$ sudo add-apt-repository ppa:kilian/f.lux
$ sudo apt-get update
$ sudo apt-get install fluxgui