Windows 7 PC will not stay in sleep mode

Hello, This seems to be a pretty common problem but I cant seem to find a solution to my case.

Basically when I tell my PC to go to sleep, it seems like its going to actually go to sleep but then my monitor will shut off and then my Tower will shut off and almost instantly turn right back on... like something is preventing it from going to sleep...

any Ideas on what might be causing this problem?

Thanks