Redshift adjusts the color temperature of your screen according to the
position of the sun.  A different color temperature is set during night and
daytime.  During twilight and early morning, the color temperature
transitions smoothly from night to daytime temperature to allow your eyes to
slowly adapt.  At night the color temperature should be set to match the
lamps in your room.  This is typically a low temperature at around
3000K-4000K.  During the day, the color temperature should match the light
from outside, typically around 5500K-6500K.

Homepage:
http://jonls.dk/redshift/
