MacBook Pro Won't Stay Asleep

When I got to work, I like to put my computer to sleep to save a bit on energy (and there's just no point in it actually running for no reason). However, if I close the lid, then the second monitor disconnects and all apps on it move over to the primary monitor, which I have to undo when i wake it. So I use the keyboard shortcut Cmd+Opt+Eject to put it to sleep. While this does put the computer to sleep, it doesn't usually stay asleep. More often than not, within a couple of minutes (usually just about 15 seconds), my computer wakes back up. It seems to pretty much always stay asleep the second iteration, though.

Any thoughts on a cause and/or fix for this issue? It's obviously not terribly critical, but it just seems like something isn't work as it should. Thanks.