Recently I started using Redshift. It is small application that changes color temperature of your screen. It is really good idea as eyes don't get tired so quickly at night.
To install it using Ubuntu type command:
$ sudo apt-get install redshift
You can also use it as system tray applet. To do so you will need gtk-redshift package:
$ sudo apt-get install gtk-redshift
Before Redshift changed color temperature:
After Redshift changed color temperature: