BI Answers: Why does your foot lose feeling when it falls asleep?
You’ve had your foot bent in an awkward position for a long time, and now it feels kind of numb, making it difficult to stand up. When this happens, we often say our foot is asleep.
That loss of feeling, also common in the legs and arms, is caused by the “temporary compression of nerves,” says Rebecca Traub, an assistant professor of neurology at Columbia University. When the nerve is pinched, it doesn’t communicate signals to the spine and brain correctly.
Nerves carry electrical signals like water through a hose, says Steven Vernino, a professor of neurology at the University of Texas Southwestern Medical Center and a fellow at the American Academy of Neurology. The signals are transmitted to the brain from a specific part of the body where they are recognised and a message is sent back down. For example, if you touch a hot stove, the nerves in your hand will fire a signal to the brain where it’s interpreted as pain, causing you to pull your hand back.
The same way that water gets stuck when you twist a hose, motor signals in your nerves can’t move if your legs are crossed or your wrist is bent. (In your wrist, a constant numbness or weakness is known as carpal tunnel syndrome.)
“One common nerve in the leg that is compressed is the fibular nerve,” says Traub, “which travels around the side of the knee and often gets pinched when people sit with their legs crossed for a prolonged period.”
It’s a common misconception that the numb sensation is caused by a lack of blood supply, says Traub. It is possible to cut off blood flow to the limb, but that’s rare, says Vernino.
The tingling or “pins and needles” feeling that people experience happens as the nerves are regaining function. Your foot or limbs are “waking up.” In medical terms, this is known as paresthesia. The discomfort, which is sometimes painful, generally causes you to change your position.
Most people can move around and relieve the pressure on their nerves. But if “someone is not able to move or sedated they can have permanent nerve injury from this type of compression,” says Traub.
This post is part of a continuing series that answers all of your “why” questions related to science. Have your own question? Email [email protected] with the subject line “Q&A”; tweet your question to @BI_Science; or post to our Facebook page.
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