Why Does My iMac Wake Itself Up?
There are a number of possible reasons, and the system actually logs the cause of each 'wake from sleep'.

Open a terminal and enter: 

syslog | grep -i "wake reason"

(or, on Sierra or newer log show | grep -i "wake reason")

This may take some time to return a result, but when it does, you should get a series of lines that look something like the following:


  Jan 24 08:52:19 hostname kernel[0] : Wake reason: OHC1


Here are the possible 'Wake Reason' codes (lifted from OSXDaily)


OHC: stands for Open Host Controller, is usually USB or Firewire. If you see OHC1 or OHC2 it is almost certainly an external USB keyboard or mouse that has woken up the machine.
EHC: standing for Enhanced Host Controller, is another USB interface, but can also be wireless devices and bluetooth since they are also on the USB bus of a Mac.
USB: a USB device woke the machine up
LID0: this is literally the lid of your MacBook or MacBook Pro, when you open the lid the machine wakes up from sleep.
PWRB: PWRB stands for Power Button, which is the physical power button on your Mac
RTC: Real Time Clock Alarm, is generally from wake-on-demand services like when you schedule sleep and wake on a Mac via the Energy Saver control panel. It can also be from launchd setting, user applications, backups, and other scheduled events.


Happy hunting!