How can I find out what is bringing the desktop out of "Sleep"?

I'm doing some remodeling and have moved the PC into our bedroom. At around 12:45am it wakes up. There is nothing scheduled nor any history of an event at that time. Some Windows services are scheduled at midnight, including RegIdleBackup. How can I find out what it is? Thank you!

 

Nice stuff, thank you!

 

Agreed - good stuff.

I have a related problem on one of my older PCs - running XP professional. Something is preventing it from going into sleep mode.

If I go into power options and set it to enter sleep mode after a short period of time, e.g. 5 minutes, it will do so. But if I set it to go into sleep mode after 30 minutes or 1 hour, it doesn't ever do it.

Any thoughts on this one?

P.S. I'm thinking it might have to do with a new wireless printer I recently purchased. I installed some software on the computer and I've noticed an "event manager" process is running. I plan to experiment to see if that is causing it, but any other ideas would be appreciated.