yep, but that isn't for the start menu button, just the physical button.
It seems to have a mind of its own. I rebooted and it came back with SLEEP as the button function. It has done that before, then changed back to SHUTDOWN at some point.
The function defined in the start menu config window is still SLEEP, but it shows SHUTDOWN when it changes. Tried changing, then changing back - no go.
Open the Local Group Policy Editor.(Type Gpedit.msc - This is not available in the "Home" releases, I'm afraid)
In the left pane, double click on User Configuration, Administrative Templates, and click on Start Menu and Taskbar.
(At this stage, I am hoping, for you, nothing is greyed out!)
Now, in the right pane of Start Menu and Taskbar, double click on "Change Start Menu power button".
Select Enable and choose one of the options in the lower pane. Apply.
See if that works long term.