IMac Awakes Unscheduled - Time Machine Updates - Reverts to Full Off

Question

My iMac is manually placed in sleep mode at night. This morning it would not awake normally. However, startup achieved only through on/off button.
Follows are the first 5 entries from All Messages (console):
8/26/08 2:45:48 AM kernel System Wake
8/26/08 2:45:48 AM kernel USB caused wake event (EHCI)
8/26/08 2:45:48 AM kernel E:[AppleUSBBluetoothHCIController][StartInterruptPipeRead] ClearPipeStall failed with error 0xe00002ed.
8/26/08 2:45:48 AM kernel [InterruptReadHandler] Received kIODeviceNotResponding error - retrying: 1.
8/26/08 2:45:48 AM mDNSResponder[23] NOTE: Wide-Area Service Discovery disabled to avoid crashing defective DNS relay 192.168.0.1:53
These messages indicate that the iMac was provoked into unscheduled awakening at 2:45 AM.
There are further entries covering the following few seconds, but no "obvious" indication when full shut down was achieved (all messages have been retained).
Are there any explanations for why this awakening/shut down took place?
Advised further steps on my part?

Answer 1

Executing "Repair Disk" from Disk Utility (bootable backup) reveals no obvious problem.
Unless this becomes a known fluke, I'll just wait until the next time it occurs.
I now suspect that this has happened at least two other times in the past 9 months. I say this because I found that it was necessary to use the on/off button to activate the iMac (which should have been in sleep mode). On those occasions, however, I failed to check the Console for confirming messages...