Battery Low [Q] My top screen darken till black

my gs3 top screen background darken slowly,
It starts as shades still cant see my op part of the background that i have set up. In a day or two the top of the background goes totally black. Note: The battery and everything else i can see them totally fine, just that my top 15% of my screen background darken till black. and the only solution is turn off my phone then everything go back to normal but this whole process start again.

what should i do??