Q: computer wakes up at night

Make sure "Wake for internet access" and "Enable Power Nap" are disabled in System Preferences>Energy Saver.

Welcome. Mine does not wake on its own so my only other thought is some third party app that's causing it to wake.

If your Mac wakes unexpectedly

Your Mac might wake for these activities:

• Network activity that uses the Wake on Demand feature. Examples of network activity include iTunes sharing, photo sharing, printer sharing, file sharing, and using Back to My Mac.
• Enhanced notifications from FaceTime, Messages, and other apps and services.
• Bluetooth activity. To prevent Bluetooth devices from waking your Mac, choose Apple menu > System Preferences, then click Bluetooth. Click the Advanced Button, then deselect "Allow Bluetooth devices to wake this computer."

If your Mac doesn't sleep or wake when expected - Apple Support

We can consider the first bullet crossed off and the iMac shouldn't have enhanced notifications.  Have you checked for bluetooth?