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
IIRC redshift was the most developed of them when I looked into it a while back.
http://alternativeto.net/software/f46lux/?license=opensource
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