Upper leg goes numb when asleep, i wake up in agony. what could cause this?

Leg numb on waking: When you are sleeping, there is reduced movements. Sometimes, one may lie in a position that compresses some nerves for hours, or compresses blood vessels that supply these nerves. These nerves then do not function properly, and there can be a sensation of numbness or tingling, even pain. With movement after you wake up, the pressure is released, and function rapidly returns to normal.

Pinched nerve: A condition called meralgia paresthetica causes painful numbness in an area on the outer thigh. It's caused by pinching of a nerve (the lateral femoral cutaneous nerve) under the inguinal ligament. Try sleeping on your sides with your knees bent, or put 1-2 pillows under your knees to keep your legs bent when you sleep on your back. Sleep in loose clothing- no briefs or tight shorts.