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. What causes our body parts to 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.
