The feeling of “needles and pins” sticking in your feet after you have been sitting with your leg curled up in one position for a long time is called “falling asleep.” What actually happens is this. Your blood usually flows freely through the blood vessels in your leg, just as water can flow freely through a hose that is stretched out. Imagine what would happen if you bent that hose, the water would only trickle through.
As your body’s blood runs through your blood vessels, one of its most important jobs is to pick up poisonous wastes from various organs. When that flow is slowed down, as by curling up your leg for a long time, these poisonous wastes accumulate and block the nerve cells from carrying messages from your foot to your brain.