Does it sleep and then wake up right away? Try unchecking Bluetooth discoverable. Here's an Apple article that suggests some basic troubleshooting steps, but be warned, sleep problems can be very hard to track down, ending often in a wild goose chase. When it's happened to me I just install Please Sleep and that has done the job immediately. It's a little program that often, but not always, works. Give that a try if you realize you're on a fruitless mission.
EDT: I see you've already read the article. Just try Please Sleep