iMac Constantly Waking: Wake reason: RTC (Alarm)

My iMac is waking several times in the night from sleep. Every time I put my iMac to sleep, moments later it wakes up I hear the hard drive spinning for about 20 seconds before going to sleep again. The screen remains off however. This happens many times throughout the night and its driving me mad because it wakes me up in the process.

I've ruled out any connected peripherals and bluetooth devices as suggested to me by someone on this forum.

I checked the wake reason in Terminal and found a huge list of this: Wake reason: RTC (Alarm). Can anybody tell me what this means and how I can fix this? I have no schedule start up or wake up. Wake for network access is also unchecked.