Just the back of my left thigh goes numb when I sleep in certain positions with my leg bent. Goes away when I straighten out leg. Nerve issue?

Yes, Nerve Problem. In the back of the knee joint, there is a depressed area called popliteal fossa. The sciatic nerve becomes tibial nerve in the popliteal fossa. It seems that the popliteal fossa is getting compressed b/c of sleeping with your legs bent. This is compressing the sciatic and tibial nerves, resulting in numbness of the back of your thigh.Thanks.