Maybe you've sat on the couch and binged your favorite TV show for six hours, or maybe you slept in a weird position, but when you tried to move your arm or leg, something happened. It felt as if you had a phantom limb; it felt like it was there but it was unresponsive and numb, and within a few seconds, the numbness turned into a combined sensation of heat and pain.
The medical name for this sensation is paresthesia, but most know it as a limb "falling asleep." It's a common sensation, unlike rare skin diseases or other bodily sensations, but not many know what causes the tingling feeling in limbs when they fall asleep - or why it hurts so much. Though it can be painful and alarming, there's usually no need for concern.
Though you may think you've somehow cut off circulation when your leg has fallen asleep, the numbness you're experiencing has to do with your nervous system rather than your circulatory system. According to IFL Science, when your body is contorted into a strange position, it's common for sensory nerves to get compressed. After some time, the brain will stop receiving signals from that limb.
When the brain doesn't receive any signals from a certain area, the area will feel numb. The tingling or “pins and needles” sensation occurs when the sensory nerves are no longer compressed, and your brain receives a rush of information.
That, however, does not mean the circulatory system cannot affect your nervous system. If a blood vessel becomes restricted, nerves may not receive the needed oxygen. In such a case, the nerves in the area will stop sending signals to the brain.
Nerves talk to the brain; they send sensory information from the body’s extremities to a central location in the organ, but different nerves have specific jobs. Numbness caused by prolonged pressure messes up which nerves are doing what, so the sensations and signals aren’t transmitted through the body correctly, according to Mental Floss.
The brain doesn't know how to process all of these conflicting signals, so it generates the sensations associated with “pins and needles."
People who have hereditary neuropathy with liability to pressure palsies, or HNPP, are more prone to feeling numbness and tingling and more likely to experience long-term consequences from nerve compression, according to Vox. HNPP is genetically passed down, and for those who have it, nerves don't respond to pressure normally; instead, they are overly sensitive.
The US National Library of Medicine says that those with HNPP don't typically endure permanent damage, though they can experience tingling and numbness for months.
When you knock what's commonly known as your “funny bone," you're hitting the ulnar nerve, which branches all the way from the spine to the tips of the fingers. Residing in a small section of the arm called the medial epicondyle that sits between the bone on your upper arm, called the humerus, and the two bones that make your forearm, the radius and ulna, the nerve is exposed without much protection from bone or muscle.
Due to its exposure, it's uncomfortable when you hit the ulnar nerve, as the BBC explains. When your arm falls asleep, it's the same ulnar nerve that’s being affected, thus the same tingling.