Any way to know what wakes an Mac up? Regularly while sleeping my Mac wakes up and i'd like to know if there is some kind of log of what trigged the waking. (Mouse click ? Keyboard ? WOL Magic Packet ? Something else ?)

I'm on late 2012 iMac with 10.8.3 

PS: "Wake for network access" is enabled.
In System Preferences under Energy Saver, is there a check in the checkbox for Enable Power Nap while plugged into a power adapter?

If so, then this is possibly why your iMac is waking up.  The description of Power Nap in System Preferences says: "While sleeping, your Mac can back up using Time Machine and periodically check for new email, calendar, and other iCloud updates".