The Drift

What’s Actually Happening When Part of Your Body Falls Asleep?

Photo by Ingram Publishing/Thinkstock, with additional illustration by Lisa Larson-Walker

You wake up from a hard sleep and think everything is excellent—until you try to move your arm. Instead of responding to the commands of your central nervous system, your forelimb, which has been trapped under a pillow or pinned between couch cushions, stays limp. With growing alarm, you realize your arm is devoid of sensation. Panicked, you remove it from beneath the cushion and shake. Nothing. “Thwack!” it goes into the mattress, or “thwack!” against the back of the couch. A stinging sensation begins from the point of contact and travels to your shoulder. You inhale with pain and exhale with relief. Your arm is finally “waking up.”

In colloquial terms, when we remain in one position too long and an arm or a leg goes numb, we say it has “fallen asleep.” But in the scientific world, this is called paresthesia. What, exactly, is happening during a bout of paresthesia—and can it ever be dangerous?

To understand paresthesia, it’s important first to understand how the nervous system works. “We think of nerve fibers that run through the body as pathways of communication,” said Lawrence Abraham, professor of kinesiology and health education at the University of Texas at Austin. “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. We might feel a thud—a mechanical transmission of force telling us we’ve run into something—but we don’t really know where we’re touching it.” A “sleeping” limb, then, is really one that’s just not in communication with the rest of the body.

Most people only experience temporary paresthesia, which happens when nerves are compressed during sleep or other long stretches of stillness. “It causes the nerves in your brain to process what’s happening at the periphery a little differently,” said Sarah Prinsloo of the University of Texas MD Anderson Cancer Center. “It’s a signaling mechanism that allows your body to say you need to readjust.”

It’s equally possible for the central nervous system to make up sensations that aren’t happening at all, Abraham added. “Sometimes this is also related to constriction of blood flow—when blood flow stops to an area, the system shuts down. When it starts to come back, you get random signals coming from the recovering system, which is the pins-and-needles feeling.”

Like most automatic processes in the body, paresthesia is designed to keep us safe; in this case it prevents tissue death. The body constantly performs micromovements (think shifting your weight in a chair or wrinkling your nose) on an unconscious level to keep it in homeostasis—maintaining its internal environment in response to external circumstances—Prinsloo explained. But when something physically keeps us from moving, our brain becomes aware of the numbness and discomfort, thereby bringing the situation to our conscious attention. Then we can perform an action—like shaking or thumping or shifting our weight—to fix the situation and prevent long-term damage.

But don’t worry: A few hours of nerve compression and decreased blood flow won’t make your arm fall off. But if paresthesia continues over a period of days or weeks, it can lead to lasting damage. “For example, people who are paralyzed or who’ve lost sensory perception sometimes get bed sores,” Prinsloo said. “That’s because those signaling mechanisms can’t tell them to move.” Repetitive paresthesia over time can also indicate bigger problem such as permanent nerve damage, she added.

“When an unusual or unexpected or scary signal comes in, we have to learn to make sense of it,” Abraham said. “It’s interesting how, when we get signals we’ve never experienced, we make up meanings for them. When you were a little kid, you never thought your leg was falling asleep. But someone at some point said, ‘That’s what happens when your leg falls asleep,’ and you start to think, ‘This sensation means my leg is falling asleep.’ ” Folk explanations aside, your “sleeping” body part is hardly napping on the job—in fact, it’s trying to tell you something quite loudly.

Read more from The Drift, Slate’s pop-up blog about sleep.