I'm using my Mac on an external monitor with a USB keyboard/mouse and the lid closed. When I put it to sleep using the remote, it switches off but then re-awakens about 5 seconds later. It's really annoying as it's usually when I'm in bed after watching a video and am too tired to get up and switch it off manually. Any suggestions about how to make it sleep? Could it be the USB peripherals making it act this way? If so, how can I stop my Mac from waking on USB?