Why Does My Arm Go Numb When I Sleep?

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Photo: Erica Shires/Getty Images

I’ll wake up suddenly with part of my arm off the bed and it’s so heavy and painful that I have to move it with my other hand. What’s going on?

You can thank the body’s competing goals of decreased muscle activity and shifting around during sleep, says Ana Krieger, MD, medical director of the Center for Sleep Medicine at NewYork-Presbyterian/Weill Cornell Medicine.

Sleep is when we recharge. To do this, our bodies go into a natural energy-saving mode where both metabolism and muscle activity downshift, Dr. Krieger says. And during a particular stage called rapid eye movement sleep, which is when people typically dream, your body protects you from acting out said dreams by inhibiting muscle activity even further — you’re basically paralyzed.

As with many other bodily functions, sometimes things go wrong here. “The first cycle of REM sleep could be very brief — it could even be less than a minute in young people — so chemically you’re getting into that stage, but you’re coming out of it almost at the same time,” she says. There could be bursts of movement that break through this mechanism, resulting in, say, your poor arm dangling off the bed.

From there, pressure on your arm could lead to reduced nerve activity and blood flow and, you guessed it, numbness and those painful pins and needles. “It’s almost like when you cross your legs for too long and say, ‘Oh my goodness, I can’t feel my legs,’” she says. “But when you’re asleep you don’t know that this is happening because you don’t have that sensation.” That is, until it gets so bad that it wakes you from your blissful slumber.

You could end up in an awkward position not only as a result of a random REM cycle burst, but also because people tend to move two to four times an hour during sleep. Dr. Krieger says such repositioning is actually thought to be a protective mechanism so that no one area is deprived of blood flow all night while you lie on it. Some people move up to ten times an hour, and we wish the best to their sleep partners. (Exaggerated movements like hitting, kicking, or jumping out of bed are more concerning to sleep doctors than lots of gentle rolling over. If you’ve been told that you do this, it’s probably worth a call to a doctor.)

The next time you wake up with a numb, dangling arm (or leg), know that your body was trying to do you a solid by keeping your muscles relaxed but also not stationary, and it just screwed everything up.

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