I realized just recently that Sleep is working on my computer, even if I change nothing in the default Power Management Balanced settings which I use and also accept the defaults in the Device Manager for my USB devices and USB hubs. It was my error to think that Sleep was not working for my computer on Windows 8.1.
What is hanging up on my computer is Hibernate, which the default Power Management Balanced settings | Advanced Settings has occurring after 180 minutes, or 3 hours, of inactivity ( Sleep triggers off after 30 minutes of inactivity ). When Hibernate occurs my keyboard and mouse shut down and the only thing which seems on is the computer itself, so pressing a key on the keyboard or clicking my mouse does nothing and I can't seem to get out of Hibernate.
What is the usual way of waking up from Hibernate in Windows ?
Even briefly pressing the Power Button, when in Hibernate, does nothing which I can see.
OTOH my computer, which is fairly old, does not support the ACPI S4 state, but only the S1 and S3 states. Could this be the reason why I cannot wake up from Hibernate ?