UPDATED March 19, 2018:
Apple Mac desktop system users (particularly Mac Mini users) that have attempted to disable system sleep or hibernation may have discovered that no matter what changes are made in the Energy Saver pane in System Preferences, when they go away for a while and come back they are greeted by a progress bar (perhaps accompanied by the Apple logo, similar to what would be seen during a reboot) and the only way to recover is to press the power button briefly (which may not always work).
What is weird about this is that in some cases the system for the most part continues to operate normally in the background. So, it is not truly sleeping. If you have music playing, it will continue to play. If someone sends you an instant message, you’ll hear the notification tone. If you click somewhere along the bottom of the screen, if and when the display comes back you will see you have launched some random application from the dock, depending on where you clicked. And so on. But still, that damn Apple logo and progress bar will randomly appear when you wake up the screen.
So, here are some ways to try to fix this, that may or may not work for you. First, open the Terminal application and from the command prompt enter this:
sudo pmset -g
Enter your password when requested and it should show you the System-wide power settings. Note particularly the values for these settings: autopoweroff, hibernatemode, and standby. They SHOULD all have a value of 0 (zero). If any of them have any other value (particularly autopoweroff), then try running this:
sudo pmset -a autopoweroff 0 hibernatemode 0 standby 0
Or you can change any of the values individually, for example:
sudo pmset -a autopoweroff 0
Also, you should increase the standbydelay (the time in seconds before the Mac will try to sleep, and yes it is in seconds no matter what Apple documentation says) to a larger value:
sudo pmset -a standbydelay 86400
Then run sudo pmset -g again and make sure your change was saved. What you are doing is disabling a specific type of sleep mode that has been problematic for many users. If you find that you system still tries to go to sleep, and you have disabled sleep in the Energy Saver settings, then you may need to do one of two things. One is to open a new Terminal window and enter this:
and just let it run until you are ready to shut down your system. Or if that does not work, try disabling display sleep – go to the Energy Saver settings in System Preferences, and move the slider bar for “Turn display off after” all the way to the right, to the “Never” setting. It will warn you that this could shorten the life of your display, but this may be the only thing that actually works. You can always turn off your display using the power switch on the display itself, or if that is in a place that’s difficult to find or hard to press, you could plug the display’s power cord into a switched outlet (such as on a small surge protector strip) and use that switch to turn the display on and off.
Note that you may want to run sudo pmset -g again to make sure that none of the above three settings have changed, if either of the following occur:
1. You make a change to the Energy Saver settings, or
2. You reset the System Management Controller (SMC) on your Mac (note that if your Mac is unplugged or loses power for more than a few seconds, the SMC may be reset).
If any of the three settings mentioned above have changed to a non-zero value, change them back to zero.
I say to TRY this because even doing all this may only reduce the frequency of the sleep states, not eliminate them entirely. The thing that finally made the difference for me was to make the changes to the pmset settings and to disable display sleep completely. There is a real bug in MacOS that needs to be addressed to truly fix this.
For more information on this, search on “autopoweroff” in your favorite search engine.