check your bios settings for what sleep modes are allow and which devices are allowed to bring the computer from sleep. Refer to your mobo manual if options are not clear (often there's very little description in BIOS itself, while manual could contain a few more helpful sentences)
my guess is you got some device that doesn't go to sleep properly and motherboard doesn't know how to deal with it.
worst case you can just disable the auto sleep function. that's what I do on my machine, it only shuts down when I tell it to.