Mac not returning to Sleep Mode: Causes and Workaround

My Mac doesn't always return to Sleep Mode. I eventually managed to understand the causes and find a perfect workaround.

I've disabled everything that might wake the machine, so now I can only wake it with the power button. Even so, it woke up approximately 10 times during the night, and 3 of those times it didn't go back to sleep. As a result, I regularly had to get up several times a night to manually put the machine back to sleep, which seriously degraded my sleep quality.

My previous Mac, a Mac Pro did the same, this is why I thought it's somewhat user-specific issue.

• Apple Support was not prepared to deal with this issue but later they encouraged me to share my solution, so here I am.
• The solution came when an AI, prompted to be an Apple system engineer, analyzed log entries and gave explanation even about the technological background.
• It turned out that disabling everything (USB, network signals, keyboard) with the PMSET command is not enough, because there's a higher level of events that affect whether the machine wakes up and also whether it gets prevented from going back to sleep.
• The potential causes are: partially malfunctioning external hard drives, keyboards, mice, and not just malfunctioning USB hubs. In my case, the actual cause was not identified but by narrowing the circle of potential causes, it is most likely the USB hub. (I need it bad.)
• There might also be a software reason preventing the device from going to sleep, for example, an application might be sending a "Don't Sleep, I'm working" signal to the operating system. You might not have control over this via some settings, and you can't wait for the external developer. I have such an application, but quitting it didn't help.
• This also means that the issue partially lies with Apple, as macOS poorly handles the behavior of certain external devices. Furthermore, users are forced to permanently use external hard drives and USB hubs because Macs have too few external ports, and USB-C often needs to be converted to USB-A.
• Therefore, this implies that the problem is unsolvable on the user-side, but I found a perfect workaround.

The solution is to issue the following command (technically a small program) every night in the Terminal application, which will put the Mac back to sleep on your behalf if it has been awake for more than 5 minutes.

sleep 10; while true; do
    pmset sleepnow
    sleep 300
done

This waits 10 seconds before actually putting the machine to sleep, and then if the Mac is awake again for 5 minutes, it puts the machine back to sleep. You can customize these timings for yourself. Okay, it could be more sophisticated but it does the job.

• In the morning, you need to interrupt it with ^C, i.e., Ctrl-C. (Focus must be on the Terminal window; red-yellow-green dots in the top-left corner, you know.)
• The following night, you can easily recall the previous command with the Up arrow key. It's a simple routine.