Ever notice how people texting at night have that eerie blue glow? Or wake up ready to write down the Next Great Idea, and get blinded by your computer screen? During the day, computer screens look good—they're designed to look like the sun. But, at 9PM, 10PM, or 3AM, you probably shouldn't be looking at the sun.
F.lux fixes this: it makes the color of your computer's display adapt to the time of day, warm at night and like sunlight during the day. It's even possible that you're staying up too late because of your computer. You could use f.lux because it makes you sleep better, or you could just use it just because it makes your computer look better. It works based on the System Time you have setup in your computer so make sure it is set to correct time.
OS X PPC users can use the old version: F.lux v11. (also, disallow updates!)
f.lux makes your computer screen look like the room you're in, all the time. When the sun sets, it makes your computer look like your indoor lights. In the morning, it makes things look like sunlight again.