It does more then just darken the screen.
It also changes the color so it is the same brightness as a halogen bulb at night, it's quite helpful and saves your eyes a tons of pain. The default night setting was too dark for me, but 3700k was perfect.
It's a little weird to get used to, but once your used to it, it's great. I tried turning it off lastnight temporarily and my screen seemed so bright and blinded me lol. When you are more tired more brightness hurts your eyes.