[brazzledazzle]

Your battery might be shot or it might not be really sleeping. Either way you can try changing it to hibernate instead of sleep using a command line utility (pmset I think). Not nearly as convenient but if your battery is dying that quickly convenience is probably secondary.

[wcarron]

Probably not really sleeping. Holds it's charge when I do more. Most of the time I just close it and leave.

[brazzledazzle]

You've probably already looked but console might actually tell you exactly what the culprit is. This has happened to me in the past a couple of times. Once with an app running in the background that I can't remember the name of that wouldn't even let the screensaver start, much less sleep. The other time was because of a kernel extension shitting the bed on a regular basis. If you're curious what non-Apple kexts you have installed you can see them like so:

kextstat | grep -v com.apple

They're at /Library/Extensions/ and you can disable them like this:

kextunload /Library/Extensions/Some3rdPartyModule.kext

Even if it's not a kext it's good to keep tabs on kernel extensions you have installed. I hope that helps and apologies if you're already aware of all that.