iMac waking from sleep intermittantly

What did you use for the virus check? Have you installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac?

First, there is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This documents describe what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community and Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support.  

There are no known viruses, i.e. self propagating, for Macs.  There are, however, adware and malware which require the user to install although unwittingly most of the time thru sneaky links, etc.   

Anti Virus developers try to group all types as viruses into their ad campaigns of fear.  They do a poor job of the detecting and isolating the adware and malware.  Since there are no viruses these apps use up a lot of system resources searching for what is non-existent and adversely affect system and app performance.

There is one app, Malwarebytes, which was developed by a long time contributor to these forums and a highly respected member of the computer security community, that is designed solely to seek out adware and known malware and remove it.  The free version is more than adequate for most users.  

Unless you're using a true VPN tunnel, such as between you and your employer's, school's or bank's servers, they provide false security from a privacy standpoint. 

Do you have Wake for Network Access enabled in the System/Energy settings pane?

Newer Mac don't need to sleep during inactivity. You can set the screensaver to to on after a set time of inactivity and then go dark after another set time of inactivity. For over night just shut it down.

Sleep is unnecessary on later Macs and should be disabled. Shut for long absences/idle overnights (no downside whatsoever) and for regular operation use the settings exactly as shown below: (reboot after setting):

Also, do not manual-sleep for brief absences; use instead the Lock Screen settings to switch the display off on timer:

Once set, you can also use Control-Command-Q to immediately lock your screen (instead of waiting for the display to switch off on timer).

soulartist wrote:

I have noticed that suddently the computer wakes on it's own intermittantly. It can wake after 20 minutes, and then not wake for many hours.

That sounds normal.

There are all kinds of things that can interrupt sleep. See If your Mac sleeps or wakes unexpectedly - Apple Support

In some cases, this can be by design. The Power Nap feature can wake the system to perform certain system maintenance tasks, as described here: Set sleep and wake settings for your Mac - Apple Support

And if you're sharing any resources, that can cause it too: Share your Mac resources when it’s in sleep - Apple Support

Virus check is clean.

You don't need any antivirus on a Mac. People get malware on a Mac by downloading pirated software or trying to access paid streaming videos or sporting events. And even then, they have to bypass multiple layers of Apple security to install malware.

I agree with Old Toad. Do not use sleep. It can cause various issues.

soulartist wrote:

I have noticed that suddently the computer wakes on it's own intermittantly. It can wake after 20 minutes, and then not wake for many hours.

It just happens. Although annoying, it's nothing to be worried about. Sometimes it seems that the slightest movement or vibration of the mouse or brush against a key can do this, other times it just happens on its own.

Highly unlikely anything in the logs or console is going to reveal any cause whatsoever, and going into them can just take you down a rat hole.

soulartist here wrote:

I used Malwarebytes; Wake for network access is toggled off. The wake from sleep started a few days ago; before then, it was never an issue. I understood it's best to leave the iMac on instead of shutting it down each night. I would think that a macOS would be "smart" enough to expose such an issue.

I agree,that sleeping can really cause strange problems, especially if the system doesn't always respond correctly to recovery. It's better to leave the computer on or set it to power save mode to avoid such glitches

In the energy pane in system settings put on the "prevent auto sleeping"

and in lock screen pane set the time to sleep in "turn display off when inactive"

As already stated many users have nothing but issues using the sleep function. No real reason to use it.

The lock screen will not prevent any application from running but if there is to be any "house cleaning" it will be done on a restart.