iMac waked up regularly
My iMac keeps waking up, although this wasn't a problem for my Mac mini, it is an issue for my iMac. The difference between is, iMac got very annoying fan noise, even no activity.
Many factors
There are so many factors to wake iMac up according to Apple, but I think the may reason is Timemachine. The option of Wake for network access in Energy Saver is for the tasks to be taken during sleep, including Timemachine. But this option is related to wake up on LAN as well, then if I need WoL, I can not turn off this option.
There is also no Do not disturb option in my this iMac, it has older macOS High Sierra.
Issue
The actual issue caused annoying problem, is Timemachine has no scheduling.
Another issue
Unlike some other OS, iMac won't maintain ssh connection after sleeped. This is a huge issue to me because I use ssh heavily. Thinking to configure VPN, and use VPN to connect to other servers, maybe can maintain connection.