Why Do Your Limbs ‘Fall Asleep’ Sometimes?

Sometimes when you wake up from sleep or get up from sitting for a long time you get an uncomfortable tingling sensation in your hands or feet. This can also be described as numbness. What causes this?

Most people think it happens when blood circulation is cut off to that part of the body. Is this true? And if so, is it dangerous?
The most recommended and detailed answers on the internet to all the above questions can be found below.

1.

Why do your limbs 'fall asleep' sometimes?

It’s not the circulation of blood (if it was, those limbs or body parts would die off) , it’s nerves being compressed and cut off . When ever you sit in a position or are in a position for a while that can compress nerves, they will stop transmitting. The nerve cells themselves will TRY to transmit at first, but when they don’t receive feedback, they stop. This is the “numbness” or “this body part fell asleep” that we all experience. When the compression ends, the nerves can sense this and all at once begin a chain of “testing” aka sending out signals to all cells affected down the line, to make sure they are still there. Its like the electrical grid firing back up. It feels painful because our nerves use pain a response. The sensory organs in our brain after about 30 seconds to 120 seconds will shut this test firing pattern off and resume normal operations. But that time in between is pretty painful. It feels like Pins and needles because each sensation is a nerve ” thread ending ” being tested.

2.

Why do your limbs 'fall asleep' sometimes?

The tingly feeling is usually related more to nerve impingement/ entrapment. It’s actually pretty hard to completely cut off blood flow to your limbs by compressing/ligating vulnerable vessels. Using the arm as an example, here is the blood supply, and the nerve supply to the upper limb. (Source: Netter’s Atlas of Human Anatomy). In the first image, note all the vessels that branch off of the subclavian, sections labelled 1 & 2. The vessels that wrap around the back of the scapula (shoulder blade) and connect with the others around the glenohumeral joint (where the ball of the humerus meets the socket of the shoulder blade) provides collateral circulation even if the subclavian/brachial artery is pinched shut in the axilla (armpit), the most vulnerable point.

Now look at the nerves. With the exception of the musculocutaneous nerve, all the nerves that innervate the upper limb pass through the ‘pit’ of the axilla that is vulnerable to compression (The MC nerve is also vulnerable, just less so).

The same applies to compression at other vulnerable points, there’s almost always better collateral blood supply than there is innervation, since taking out nerve supply upstream cuts off the downstream more effectively than with blood flow.

I was a a second (or 1st?) year med student at the time so I’ll expand a little – I focused on macro scale blood flow in that answer & waved the rest away as “nerve compression”, but what actually happens with nerve compression? It’s likely related to ischemia/impaired flow, but at the microvascular level supplying the nerve. The nerves have tiny blood vessels (vasa vasorumnervorum) that supply them, and compressing the nerve will compress these and can compromise the blood supply to the nerve.

This abstract has a decent overview, relevant part quoted below:

while the membrane depolarization produced by ischemia affects both transient and persistent Na(+) channels. Postischemic and posttetanic paresthesias occur when hyperpolarization by the Na(+)/K(+) pump is transiently prevented by raised extracellular K(+). The electrochemical gradient for K(+) is reversed, and inward K(+) currents trigger regenerative depolarization. These mechanisms of paresthesia generation can account for paresthesias in normal subjects and may be relevant in some peripheral nerve disorders.

The gist of which is that 1) cuteneous & superficial sensory nerves are more likely to have “false” firing (see section above where I quoted) not related to actual stimuli; 2) ischemia (lack of blood flow) causes tissue pH to lower, this is balanced by exchange of Hydrogen ions outside cells for potassium ions inside such that Potassium in the fluid is higher. Then when this hits a tipping point, potassium flows back into the cells – including the nerve. This triggers depolarization (aka firing) of the nerve.

Typing this out between patients, please let me know if you have any questions or want more explanation of something! Hope this answered your question.

3.

The pins and needle feeling is due to external pressure on a nerve. I want to expand on it a little bit. There are a couple of reasons why the pressure affects the nerves. The most common reason for it is that the compression on the nerve temporarily cuts off the blood supply to that part of the nerve. This results in a reversible injury to the nerve. The numbness as well as the pins and needle sensation is called a paresthesia and the nerve injury is called a neurapraxia.

Now for the reason why you feel pins and needles. What it actually is is your body’s perception of pain from the nerve. Nerves themselves are actually composed of a group of axons and a bunch of supporting cells. An axon is an offshoot of the nerve cell (the neuron) and this axon is what brings the signal from one neuron to another. These axons all send different signals. There are certain axons that come from neurons that control movement. There are other axons that come from neurons that control normal sensation and other axons that conduct the signal for pain. When injured, these axons all recover at different rates. The rate of recovery is dependent on a number of factors including the number of supporting cells and the axon’s size. It just so turns out that the axons that recover from the compression the fastest are the pain fibers and this is why you get the sensation of pins and needles first before you get any other sensation back.

Why do your limbs 'fall asleep' sometimes?

Most of the time, the limb that fell asleep is temporary and harmless but if the pressure is on for an extended period of time, it can cause a long lasting or even permanent injury. This is why, when a person undergoes surgery, the people working in the operating room take a lot of care in ensuring that no part of the body is being compressed too much.

Source

You may also like...