What is a sleeping foot and what causes it?

Here's a vivid summary of "Sleeping foot":

Sleeping foot occurs when the sciatic nerve in the back of your thigh gets temporarily squeezed against your femur (thigh bone). This usually happens when you sit on a hard surface, like the edge of a chair, for too long. The compression causes a tingling, numb sensation, or "pins and needles" (paresthesia), in your lower leg and foot, making it feel like your foot has "fallen asleep."