Waking up to a prickling sensation—or just complete numbness—in your arm is startling to say the least. But why do our limbs “fall asleep” in the first place? According to primary care physician Marc Leavey, it’s all to do with your nerves and not, as many people believe, the blood flow to the limb in question.
“The feeling you have in your skin is transmitted by nerves […] if you apply enough pressure to those nerves, it will interrupt their function,” says Leavey. In other words, if you fall asleep on your arm, or sit on your own foot for a long time, the pressure put on those nerves is enough to stop them transmitting effectively. It works, as Leavey explains, a lot like a telephone wire: “Your phone service would go out if you were to cut the telephone wire, just like your nerves temporarily ‘go dead’ when you apply enough pressure to them.”
This happens most often in our arms and legs because we have large nerves located in our thighs, shoulders and elbows, which branch out to nerve endings in our hands and feet. The numbness often centers in our extremities as the nerves aren’t able to transmit sensations past the blockage higher up. These large nerves in your elbows also account for the unpleasant sensation of bumping your funny bone—the ulnar nerve runs just under the elbow skin, where it’s extra exposed.
As for the sensation of pins and needles as the limb returns to life, that’s a result of the nerves attempting to regain their connection after being cut off. “Think of how your TV picture is pixelated as it works to regain service—that’s kind of what this is like,” Leavey says. In other words, your nerves work at half capacity while they’re rebooting.
Next time you wake up feeling like your arm is possessed by a demon, just give your nerves a little time to reboot and you’ll be fine. Unless, of course, your arm really is being possessed by a demon, in which case, we recommend the holy water.