the exact same thing just happened to me!!
i was trying to lay still so my body would fall asleep.
well i could not move any muscle in my body..so I know it was SP...but no dreams were coming to mind and i tried to think of one but my mind became unfocused. i could still hear the buzzing of the power bar in my room..and i heard my mom come in the side door...and i was just laying there...it was boring!

i tried opening my eyes...with some force...and they felt like my physical eyes and everything in my room was the same..

this has happened twice now ...with no LD! what do I do after i know SP has kicked in?