I could either have the brightness turned up so I can see, but have my retinas blasted with excess photons, or have the brightness turned down, and then squint as I try to read.
Instead of messing with the brightness of your monitor, f.lux changes the colors of your monitor to be easier on the eyes. At first, it was kind of an uneasy yellow color, but my eyes adapted quickly.
Everything felt normal, until I turned f.lux off. My monitors colors returned to normal, and I instantly felt like icepicks were being driven into my eyes. Needless to say, f.lux was turned back on, and I use it every night. I heart f.lux.