It’s an all-too-familiar feeling … waking up from a peaceful slumber to discover that an arm or leg is totally, immovably asleep. Once you struggle to get the limb moving again, you experience the uncomfortable pins-and-needles sensation of restored feeling, plus the humiliation of swinging your arm or leg around as if it was a heavy dead weight. Why do limbs fall asleep, and, like other annoying body quirks, should we worry about it?
A Fleeting Loss of Feeling …
What we call “falling asleep” is called paresthesia by doctors and medical experts. Paresthesia refers to any kind of numbness, tingling, or prickling, whether it’s transient or chronic. Most of these sensations are short-term, like the discomfort from sitting or lying in the same position for too long, and they resolve on their own. Certain body positions, like crossed legs or a head resting on an arm, can put pressure on nerves or blood vessels. Constricting the nerves interrupts the electrochemical impulses that control our movements and sensations, and pressure on blood vessels can cut off circulation to the affected area. The result is that signals are scrambled or can’t get through, and the brain has a hard time communicating with the affected limb, both to send it instructions and to receive sensory information.
Paresthesia is most likely an evolutionary adaption that signals us when it’s time to change position and get things flowing again. Once we begin moving a sleeping limb, the blood begins to flow again, and the nervous impulses have a clear path. As our bodies adjust to the newly-increased blood flow, we experience the familiar pins-and-needles feeling, which can range from a gentle tingling to uncomfortable burning. The physical sensations change because as nerve conduction and circulation are restored, certain cells react faster than others, causing feeling to return in a predictable fashion. The nerve fibers that sense pain and temperature are very sensitive, so those are the first sensations to return. The next cells to respond are the muscle fibers that allow us to control our movements, so we can begin to move easier even though the limb isn’t yet sensitive to touch. The last sensation to return is touch, because tactile nerve fibers have thicker sheaths, and it takes them longer to begin transmitting impulses.
Or a Serious Symptom?
Most paresthesia is fleeting and temporary, and like cold weather and tight clothes, can cause reduced sensation. Even tightly-tied shoes can cut off circulation to the feet. However, when these sensations last a long time or pop up during normal activities, it can indicate a more long-term problem with the nervous system. They could indicate a pinched nerve, a herniated disc in the spine, fibromyalgia, or diseases like shingles and diabetes, all of which can cause pain and sensory loss.
Intermittent numbness can also be a sign of more serious problems like Guillain-Barrè syndrome, carpal tunnel syndrome, or heavy-metal poisoning. Paresthesia can also be a predictor of an aneurysm or stroke, or a symptom of long-term problems like multiple sclerosis or hypoparathyroidism. Although transient or pressure-related tingling or numbness is generally harmless, a neurologist should investigate paresthesia that arises during exercise or normal activities, affects the fingers, toes, and lips, lasts for a long time, or is accompanied by pain or swelling.
For most of us, numbness and tingling only cause us only mild discomfort. When a limb falls asleep, it’s tempting to ease it back into use, but taking it easy can actually drag out the discomfort. Vigorous massage or use of the limb will cause blood to flow faster, returning function sooner. For sleeping arms, try making fists and squeezing to allow blood to flow into the hands, and for legs, try pointing and flexing the feet to encourage circulation. It might be momentarily more uncomfortable, but the pain won’t last long. All we can do is drag our useless limbs around, trying to get the circulation back sooner rather than later. Because those pins-and-needles hurt.