Why Do Limbs 'Fall Asleep'?

At some point, we've all experienced the sensation of "pins and needles"—that strange numbness that comes from applying pressure to our arms or legs. If you've ever wondered what causes it, or if it's dangerous, wonder no more.

We've got nerves running through our bodies that act as lines of communication between the brain and the other body parts, transmitting commands from the brain and relaying sensory information back to it for processing. With a "sleeping" limb, your nerves are going a little haywire because prolonged pressure has actually cut off communication between that limb and the brain. (The tingling sensation is technically called paresthesia.)

Pressure puts the squeeze on nerve pathways and blood vessels, so the nerves can't transmit signals properly, and the blood vessels can't bring oxygen and nutrients to the nerves. The cutoff interferes with the normal flow of information between the limb and the brain, and the signals going back and forth get jumbled. Some nerve cells stop sending info entirely, while others send impulses erratically.

The problem is compounded by the fact that our nerves are pretty specialized, and different kinds of nerves and sensory receptors receive different stimuli and transmit different information. (We've discussed another bodily oddity caused by this.) When the various signals get scrambled and aren't transmitted normally, the brain starts to misinterpret the info it's getting and generates an array of sensations, like warmth, numbness, and that tingling feeling.

When a limb falls asleep, we usually try to "wake it up" by changing positions. Blood flows back to the limb, giving a little boost to the misfiring nerves and making the tingling seem worse, but eventually the nerve signals begin to flow properly again. The pins-and-needles sensation is annoying for a few minutes, but it's a nice little prompt for us to relieve the pressure on a limb before serious nerve damage occurs.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.