My legs fall asleep if I sit more than 10 or 15 minutes at a time, unless I elevate them. But if I get up and move around every 10 minutes or so, it doesn't happen. Lying down, with my feet elevated, also makes them feel better if I get to the point that they do fall asleep. My doctor ordered a test that checked my circulation to my feet, and didn't see a problem. I thought it had something to do with Neuropathy, and sitting maybe was pinching the nerves to my legs.
Does this happen even when you are sleeping? Maybe you just need to elevate them a bit, when you lie down.