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its when oxygen poor blood cant get back to the heart and oxygen rich blood cant get to the limb that is asleep, hence the reason you feel all tingly cuz your limb is technically "shutting down" cuz o lack of oxygenated (sp?) blood.
i hate the feeling. iv woken up with both arms asleep and i sriously couldnt move. it was the scariest feeling ever cuz i couldnt use either of my arms to get out of bed. i finally had to just roll off my bed and lay on the ground until the feeling came back.
You don't feel it goign to sleep, only when you move it again and get the 'feeling' back in your limbs. You cut off circulation to your body part, and no oxygen rich blood can get there. Your cells start to die. When you move and the blood flows back in, it's basically bringing them back up to speed.
It's why heart attacks are so bad, the heart stops pumping and no fresh blood +oxygen gets to it, and the tissue starts to die