Why Our Limbs 'Fall Asleep' And Why It's So Painful

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.

Paresthesia is a common symptom of pregnancy, and there are a couple of reasons why pregnancy causes numbness and tingling in a person’s extremities. The hormonal change in a pregnant person's body is the main culprit. When pregnant, the body produces relaxin, a hormone that loosens ligaments for pelvic expansion to occur. In can have adverse side effects, though; when too much relaxin is produced, other joints in the body also become more flexible and collapse at points where stress can be placed on peripheral nerves.

Pregnancy can also cause paresthesia solely in the lower body, according to Dartmouth Medical School. Similar to when a person gains weight or wears a tight belt, there is a chance that a nerve which runs from the brain down into the legs - the lateral femoral cutaneous nerve - can get caught under a ligament in the pelvis.

As if hot flashes weren't enough, menopause can also cause paresthesia and peripheral neuropathy, according to 34 Menopause Symptoms. Menopausal women may have enlarged blood vessels that could press onto nerves and cut off circulation. 

Postmenopausal women are particularly at risk for carpal tunnel, a form of peripheral neuropathy localized in a person's hands because their wrist structure becomes enlarged and places undue stress on the nerves.