Any way to know what wakes an Mac up?
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".