Why do hands, arms, and legs fall asleep?

There are many possible reasons why an arm or leg might fall asleep, says doctor and neurology researcher Martin Syvertsen Mykland at NTNU.

"It most often happens when you've been sleeping, sitting, or lying on an arm in a certain position," he says.

You have probably heard various explanations for what happens. One of the most common claims is that blood flow is cut off. But that's mostly a myth.

Two types of nerve fibres

We need to look at the nerves.

First, a brief overview of how they work. Nerves are described by the Great Norwegian Encyclopedia as pathways for impulses in the body.

Sensory never fibres carry sensory impressions from different parts of the body back to the brain.

Motor nerve fibres, on the other hand, send signals from the brain out to the muscles. Nerves are enclosed in protective casings called nerve sheaths, which help transmit signals quickly.

Most of them contain both sensory and motor fibres.

This helps explain why you can lose both feeling and the ability to move, for example, in a hand that has fallen asleep.

Signals get blocked

"It's usually not because a major blood vessel is cut off, as many people believe. Rather, it's thought that the pressure affects blood flow within the nerve itself. It’s still unclear whether that or the direct mechanical pressure on the nerve is the main reason you lose sensation," says Mykland.

When a nerve is compressed, its signals are temporarily blocked.

What happens inside the nerve depends on how long the pressure lasts and how intense it is, the researcher explains.

Can last for weeks

If the pressure is brief, things return to normal quickly.

But if the nerve is compressed for a longer time, it can sustain more significant damage, says Mykland.

"It can take several weeks for sensation to return to normal. Still, normal function comes back because the nerve itself hasn’t been permanently damaged," he says.

"With short-term pressure, it's probably just the outer layer of the nerve that prevents the signal from being transmitted normally," he explains.

Numbness and weakness

You may have noticed that after an evening of heavy drinking, it's easier for a hand or leg to fall asleep.

Mykland says this is called Saturday night palsy.

"Alcohol makes you more susceptible, and you tend to sleep more deeply with your arm in an awkward position, often for a longer time," he says.

This can lead to a type of injury that takes longer to heal. It may take a few weeks before you're back to normal.

In such cases, it's the radial nerve that gets compressed. It's one of the three main nerves in the upper arm. It runs from the neck, around the upper arm bone, and extends all the way down to the fingers.

"It can cause both numbness and weakness," he says.

If the nerves supplying the entire arm are compressed in the shoulder, you may feel symptoms throughout the whole arm, Mykland adds.

A puzzling tingling sensation

As feeling slowly returns to your hand, you may notice a kind of electric, prickling sensation. This is often called pins and needles.

Exactly why this happens isn’t fully understood, says Mykland.

"It often comes along with numbness. Sensory signals are reduced, but why that creates a tingling feeling is still unclear," he says.

But it does relate to changes in nerve signals, says Mykland. For example, it may affect how quickly signals are transmitted.

Some may need surgery

Some people experience limbs falling asleep more frequently than others. There's no clear explanation for this either, says Mykland.

Sometimes, the body simply doesn’t leave enough space for the nerves. This can also be hereditary.

"If you frequently sit with your elbow bent, for example, you might feel numbness in your little finger. The nerve passes through a very tight space there," says Mykland.

Carpal tunnel syndrome is another common condition. In this case, the median nerve becomes compressed as it passes through the wrist. A minor operation can resolve the problem for most people, according to the NHS.

"Use common sense"

Certain conditions can also make nerves more sensitive to pressure, Mykland notes. So when should you get it checked out?

"If symptoms persist, cause weakness, or are bothersome, it may be worth seeing a doctor," he says, adding:

"Most of the time, common sense is enough. These episodes are usually temporary and harmless. It’s rare that they’re caused by an underlying condition."

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Translated by Alette Bjordal Gjellesvik

Read the Norwegian version of this article on forskning.no

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