This happens to me every night when I am giving my newborn son a bottle. Inevitably, he falls asleep while eating, I fall asleep while feeding, and wouldn’t you know, the arm on which his head is lying falls asleep too. I used to think that this was because when you put pressure on your limbs (such as your arm or foot), you prevent blood from flowing to that limb, creating a tingling sensation and an inability to move that part of your body for a minute or two.
As it turns out, your nerves are more to blame. See, when you put pressure on a certain part of your body, you are squeezing nerve pathways in the process. When these nerve pathways are unable to transmit messages from your brain to parts of your body, then that part of your body “falls asleep” because it is unable to communicate with the brain.
When you release that pressure (like when my baby wakes up crying because I’ve been pointing the bottle at his now-drenched sleeper instead of in his mouth), then your limb starts to “wake up.” This process is usually accompanied with a tingling sensation, which is the nerves in your limbs reactivating their communication with the brain. Once normal communication between the brain and body part resumes, then that tingling feeling goes away.
This sensation isn’t just annoying; it actually serves a very important purpose: to alert you that something isn’t right. If your foot falls asleep for 10 minutes, no biggie. But if you were to cut off communication with the brain from a limb (and blood circulation as well) for a prolonged amount of time, then you could cause permanent nerve damage to that limb.
The clinical term for this sensation? Paresthesia. It’s often described as the “pins and needles” sensation you get when your limb falls asleep. Though most often a transient condition, paresthesia can be a chronic condition too — as a result of a demyelinating disease (such as multiple sclerosis, where the myelin sheath around the nerve fibers in the central nervous system deteriorates) or of neuropathy, where neurons are permanently damaged as a result of an injury or an infection. This kind of paresthesia doesn’t come on as a result of an awkward limb placement, but as a result of the underlying issue.
Usually though, the numbness and tingling you feel is just the result of an awkward sitting or sleeping position and resolves itself as soon as you arrange yourself differently. All you have to do is try shifting your weight a bit — then give it a minute. And remember — the “pins and needles” sensation you’re feeling, though irritating at times, is actually a very good thing.
Other forms of paresthesia
While the most common way of temporarily experiencing the "pins and needles" sensation is by crossing your legs or arms for too long, there are other forms of paresthesia that can have long-term effects and some that are instantaneously.
Sometimes, it's possible for tissue to apply pressure to our nerves — like with carpal tunnel syndrome. Many people who frequently use computers suffer from this syndrome, which occurs when the inflamed tendons apply pressure in the wrist and causes the tingling feeling in the hands.
While carpal tunnel can last for months or even years, some paresthesia lasts only a few seconds. A common one is when you hit your "funny bone" (a.k.a. the ulnar nerve in your elbow). The nerve runs from your shoulder down to your pinky and ring fingers, but it's especially sensitive around the elbow because it isn't protected by flesh. So when you hit that particular part of your elbow, your ulnar nerve sends a rapid signal through the nerve.
Also having a love for spicy food can cause the similar tingling sensation in your mouth. That feeling you experience when you bite into a pepper is caused by a pain receptor reacting to capsaicin, the active chemical in chilis. When it's activated, it tells your brain that your mouth is too hot.
Editor's note: This article has been updated since it was originally published in January 2012.