posibly a pinched nerve or positional. Both my arms go to sleep at night. I have to put pillows under them. Nothing comes back in the test but that doesn't mean it not happening. I wish I had some answers for you but that what the doctors told me. Sorry.
sleep on your side with a pillow between your legs...
also, do some hamstring stretching everyday to actually stretch our the nerves in your legs so blood and sensation keeps flowing into them during the night...
Sounds like a form of circulation problem maybe. I would ask your doctor to prescribe a vascular test on your legs to rule out any hidden blockages.