bightness problem

hey mac users! OK so not too long ago i purchased the beautiful 27 inch iMac for my room and it's amazing!! but as of two days ago....it started doing a strange thing and I'm wondering if i can get some help as to what i should do

it seems as though when the computer wakes up from being in sleep mode it's fine ...but a few minutes later the screen gets slightly darker...but only on the left half! so i've been having this two tone display and i can't say i like it that much .....but is this a hardware problem? software? setting? i dunno....

Sounds like a hardware issue. Where'd you get it? It's under warranty if it's less than a year old, for sure. Did you purchase Apple Care?