There are in fact a number of free solutions...

IIRC redshift was the most developed of them when I looked into it a while back.

http://alternativeto.net/software/f46lux/?license=opensource

Nice! Thanks for the tip:

  dfc@ronin:~$ apt-cache search redshift 
  gtk-redshift - Adjusts the color temperature of your screen with GTK+ integration
  redshift - Adjusts the color temperature of your screen

Another thanks, the benefits are instantaneous, incredible.

Side note about f.lux. I installed f.lux on my windows XP laptop and it kind of messed up the settings. The funny thing is even after uninstalling f.lux, my laptop retained the brightness settings that were set by f.lux. (Thankfully, it is fixed now.) It also crashed a few times due to its memory consumption.

Issues highlighted in the FAQ page here. http://stereopsis.com/flux/faq.html