It turns out that this problem was caused by the .2932 (latest) driver version. When I decided to try "Roll Back Driver" at Device Manager -> Intel HD Graphics 4000 -> Properties I was rolled back to the .2768 version of the driver and I found that the sleep mode was then available. I did this because I noticed a yellow warning icon next to the Intel adapter icon in the Device Manager which also disappeared with the roll back. This icon recurred when the latest driver was reinstalled again by Windows Update.
Because I ran Windows Update immediately after installing the Win8 OS on my new system, and since I never saw that sleep mode was available I had no idea of the source of the problem initially and only eventually figured it out after running powercfg:
> powercfg \availablesleepstates
An internal system component has disabled this standby state.
EDIT: I later found that driver version .2875 works and supports sleep mode and OpenGL version 4.0, whereas OpenGL Extensions Viewer showed that .2768 version only supports OpenGL version 1.1.
Message was edited by: cattledogit
If the previous graphics driver is working out for you, then use that version for now until we come out with a newer graphics driver version, but please keep in mind that our graphics drivers are generic, so it could be your system needs to use the original drivers give only by your system manufacturer.