My iMac turns itself on during the night
How do you stop it (the Mac) turning itself on and staying on - it does not go back to sleep
iMac 27″ 5K, macOS 10.15
Mine is simply turned off when I go to bed, and back on when I get up in the morning. Been doing this for years without an unplanned night light event. If your sleep schedule is at night, your arriving email and messages will be there in the morning without the Mac running all night.
I live on a busy street and during the day, some heavier truck traffic causes a vibration that effects the Magic Mouse II, and wakes the Mac. Presumably, you do not have wake for network access set in System Preferences : Energy Saver, or start up automatically after a power failure.