Display fading/brightness reduced while on full screen

Hey ! I am using a Sony Vaio lap and i've been experiencing the exact problem. It is really annoying to watch my screen fading/ reducing brightness by itself while watching movies on Full screen or the same thing happens with a dark wallpaper while on desktop. I couldn't find any setting that i can change. I do not get this problem while my laptop is plugged in. What should i do now? Any idea?