I have had a problem with "mac won't sleep" since I upgraded to High Sierra. When I ask the computer to sleep it shuts off for about 3 seconds and comes right back up again. Also, I have the settings to put my computer in sleep mode after 3 minutes of activity, and that is not working either. I have tried to re-install Sierra, but apparently that can't be done, as I purchased my imac with El Capitan, (january 2017). I hate this high sierra, it's just nowhere near as good as the last few upgrades.