Why does your arm or leg "go to sleep"? I'm curious - what causes a persons limb to go to sleep. I imagine it has something to do with a persons circulation, but I've never been given the exact answer.

Pressure. Pressure on a nerve or pressure on a blood vessel will make a limb "fall asleep" a nerve has blood vessels on it and the acute pressure affects the circulation to the nerve. The nerve cells also are sensative to pressure as deforming the cell membrane interferes with the cellular chemical reactions that transmit the nerves electrical signals. The article I reference below gives a nice more detailed answer as well.