F.lux is a tool that changes the color warmth of the display to match our surrounding light and time of day. During day, it is bright white-blue to match the bright sunlight our eyes can see all around. At sunset, it switches the display to warm-red to match the warm glow from incandescent or flourescent lights we use after sunset. F.lux uses the latitude and longitude coordinates of our location to make this switch from day to night as precise as possible.
F.lux has a simple Settings dialog. I found the default Daylight setting of 6500K to be okay, but the default for night is Halogen (3500K), which looks extremely warm to me! F.lux might have set Halogen as default since USA still uses a lot of light bulbs instead of flourescent lighting. That is not the case in Asia, so I chose Flourescent which is 4200K. I also chose the Transition Speed to be slow, at 60 minutes. Sunset here at Singapore is around 1900, so starting at that time F.lux starts to slowly increase the warmth of the display until it reaches the set maximum value by 2000.
I have been using F.lux for a few months now, on both my work and my home displays. It has become a must-have tool for me! You may want to temporarily turn it off (Disable for an hour) if you are processing photos. The warm colors of F.lux at night take a little getting used to in the beginning. Since I am used to it now, my eyes strain when given a normal white-blue display at night! My eyes are no longer strained when I look away from the display to the lit surroundings.
Research is finding strong connections between the light our eyes perceive at night (and day) and our sleep and mood. F.lux is the result of this research for our computer displays. Lights like the Philips Wake-Up Light are appearing on the market now to provide natural transitions from and to sleep in the mornings and at night.
For Linux: I was glad to see that there is a F.Lux version for Linux here. However, it does not seem to work on Ubuntu 14.04. The display does not change no matter what longitude-latitude is set and the Preferences option does not open up at all.