Why is my 2017 imac restarting (bought in 12/2018)?

Restarts can be caused by several different scenarios.

Random restarts can be caused by brief power outages or disruptions on your local electric grid during the night. To prevent the iMac from auto restarting after the loss of power, go to Apple > System Preferences > Energy Saver and make sure that "Start up automatically after a power failure" is un-checked. Then if you find the iMac off instead of sleeping, it likely encountered some type of external power loss during the night.

Regular nightly restarts that occurs at the same time every night are caused by a set schedule that occurs at the same time every night. To check for a set schedule, go to Apple > System Preferences > Energy Saver > click on the Schedule... button and make sure that nothing is checked or setup that would cause the iMac to Auto Restart.

I suspect a fail(ing) power supply. Your 2011 is too old for an Apple Store to repair it however you may find an independent Apple Authorized Service Provider that can. I would not recommend doing this repair yourself as it can be very dangerous if you don't know what you are doing.

If you would like to, please post an EtreCheck report of the computer and we can look for any software issues.

Please post an EtreCheck report of your system and we can look for any obvious issues. Please click the link, watch the slideshow to learn how to use the tool and download and run the free version of the app and then post the complete report it produces as your reply to this message. 

When you have run the report and are ready to reply to this message please attach the report. You will see a screen shot below that indicates where to click in your reply message.