Waking up in the middle of the night to discover one of your arms has lost all feeling is frightening.

At first, the limb is limp and flops around like a useless bag of bone before coming back to life with a flood of “pins and needles” sensations.

When this happened to me as a kid, I panicked, thinking I’d done something horrible to my body, anxious that I’d never be able to move my arm again. But the feeling in my arm always came back.

This phenomenon is really common, says James Dyck, a neurology researcher with the Mayo Clinic. And it’s actually a cool example of how the body can protect itself even during the paralysis of sleep.

Dyck explained there’s a common misconception that pins and needles and numbness are caused by a lack of blood flow to the nerves. “The more likely thing is nerve compression — nerves are being pushed on and squashed, and that causes these symptoms,” he says.

You have several nerves in your arm. Each serves a vital function.

 Am Fam Physician

The axillary nerve lifts the arm at the shoulder.

The musculocutaneous nerve bends the elbow.

The radial nerve straightens out the arm and lifts your wrist and fingers.

The ulnar nerve spreads your fingers.

Although Dyck says the exact physiology isn’t completely understood, the effect of compressing any of these nerves in sleep — when you sleep on top of your arm or pin it underneath a partner — is like stepping on a garden hose. The information that flows from your extremities back to your brain is temporarily disrupted.

So why does it feel paralyzed upon waking?

Dyck suggests two reasons.

1) It is actually, temporarily, paralyzed. During REM sleep, the brain sends a signal to cause a body-wide paralysis. The purpose of this Read More Here