I have used the 'sleep' shutdown option a couple of times and did so last night but this morning it hung with the HDD light on continuously ~ hmmmm!
Pressingthe reset did not work, nor did forced shutdown (4 secs on the power button) and cold start....................thinks, I turned off the PSU switch for 30 seconds to allow everything to discharge.
Turned on again and booted up fine but interestingly into "resume" so 'sleep' runs in copying the state to the HDD not into RAM - sory if I have stated the obvious and without checking I think there is BIOS setting to select RAM or HDD sleep???
Hope that is of some help to other folk with the P55A-UD4 motherboard