screen light seems to dim on its own? really?

It changes based on the ambient light.

 

it has an ambient light sensor next to the camera. when the ambient light darkens the screen brightness darkens to conserve battery.

edit: hello Skil

 

If you leave the box unchecked you won't have that problem anymore.

 

id suggest Not turning off the brightness option.
it only adjusts so it can have a better visual for you.
think about it
if theres backround light with more light from notebook. thats bad for your eyes as well as in certain cases it will cause you to have a headache.

 

I personally like to have the option of doing it myself. And no headaches so far
It's just a matter of personal preference lol

 

I had to shut it off on my Unibody MB because when I use the laptop while watching TV with the lights on dim, the TV reflecting off the wall behind me was enough to cause the sensor to change the screen brightness.

It was REALLY annoying, when watching certain shows, the screen would be changing brightness levels every 2-5 seconds even it seemed!!