Why Do Our Limbs Sometimes "Fall Asleep"?

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You know the feeling: You sit cross-legged or sleep on your arm for a little bit too long and it feels like your limb has been replaced by a rubbery, floppy peg. So, what’s this all about?

Most of us say this a limb "falling asleep” or a “dead leg”, but scientists call it paresthesia, which translates from Greek to mean something like “besides sensation”.

You might assume it's simply blood being cut off from your muscles, but it has more to do with the nerves. Sensory nerves are a bit like a bundle of cables that carry sensory information through electrochemical impulses to your brain and spine. However, if enough pressure is pushed onto one of these cables, then they find it tricky to fire and send signals. 

Simply think of it like a garden hose. If you step on it, the flow of water stops. If you pinch the nerve fiber, it can result in the electrochemical impulses hitting a kink. 

“We think of nerve fibers that run through the body as pathways of communication,” Lawrence Abraham, professor of kinesiology and health education at the University of Texas at Austin, told Quartz in 2015. “We gather information and bring it to our central nervous system from specialized sensory receptors all over the body. If there’s compression of the sensory nerves, we simply don’t get any information, so it feels numb.”

Once the numbness has gone and your sensation returns, you’ll often experience pins and needles. This is essentially your nerves firing off and panicking, still a bit confused from what they just experienced.

All of this can also occur if the limb’s blood vessel flow is cut off because the nerves no longer have the necessary oxygen to carry out their job. However, the blood is just part of the story. Think about the sensation of hitting your “funny bone”. You get pins and needles when you hit your elbow because the pressure is being applied to the ulnar nerve, the nerve that travels between your fingers and arm. As there’s not much bone protecting this cable at the elbow, a quick tap can cause tingles to shoot down to your hands.

Getting the odd dead leg or pins and needles is nothing to worry about, but if you find yourself having chronic paresthesia, it can be a sign of a serious neurological condition. Most of the time, however, it's nothing that a quick hop around the room won't remedy.


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