My Dell Optiplex 755 SSF turned itself on every couple hours, especially in the night.
As the hard drive always started up and 2 seconds later was powered off, I figured it could’t be good for the drive.
To remedy this, you have to turn off Intel-AMT.
At boot, when the DELL logo comes up, press STRG+P. You now come to the Intel-AMT menu.
First off, you need to change the password. It needs 8 characters, at least one capital letter, a number and a special character (?!;). Note it down!
Now, navigate to Intel ME State Control and simply disable it. Save and quit.
Here is a description from Intel in German: