Holding / pressing the power button in most cases causes the machine to....just shut down, when you reboot you will get that excellent Windows did not shut down properly...sometimes that is good, select start windows normally and all should be good.
However you can allocate the power button on a single short press to shutdown your machine, most useful. And you still have the option, press and hold and get a hard shutdown, start windows normally, enter repair or safe mode.
Sleep and hibernation have always been a problem in windows so on my machines are not enabled. Hibernation can and will take up some space but it can be disabled quite easily;
As admin, open CMD and type the following command:
powercfg.exe /hibernate off
To reverse that the powercfg.exe /hibernate on
In my experience sleep and hibernation do cause problems in everyday computing and these options should not be used