Waking up in the morning is sometimes accompanied by the refreshing sensation of one's hand being stabbed thousands of times by tiny needles. What exactly causes the "pins and needles" sensation? And is the sensation ever dangerous?
Pins and Needles
Our morning routines are occasionally enlivened by a little dance. A person curls up and alternately pats at or desperately shakes one of their hands or feet. This is spectacularly funny to the onlookers, but not quite so much fun for the person doing the dancing, who has a limb that feels both numb and covered with repeated stabs of pain. We've all been there.
When I was a child, I was told that the sensation of pins and needles was caused by the circulation to that limb being cut off. The limb was reviving, and this caused pain. This worried me, as I knew that I slept for hours, but was told that cells, particularly brain cells, when not supplied with blood and oxygen, would start dying in about five minutes. Practically speaking, it seemed odd I had coincidentally woken just before my entire arm died while I slept.
What's Actually Happening
There are quite a few causes of the sensation of pins and needles. Some are serious - and if you regularly wake to extreme pins and needles you might want to go to a doctor - but they all stem from two sources. One is direct pressure on the nerves, causing them damage over time.
The other is a cut in blood flow to the nerves - although usually not a complete loss of blood flow to the limb. When you sleep on an arm or a leg, you compress all the blood vessels in that limb and those vessels have trouble pumping blood around the area - but you are not wearing a tourniquet. Blood still flows through the limb, but it's often not quite enough blood. Nerves are great consumers of energy and oxygen, so they feel the lack of oxygen before other tissues. This is why it's relatively hard to "kill" a limb, and relatively easy to kill the brain. A few minutes of lack of blood will kill the neurons the brain, but other cells continue to live for anywhere between one and six hours without a decent blood supply.
As nerves begin to take a little damage from lack of blood, they start firing off at random, especially the nerves that deal with pain. Most of the time we feel this right away, and shift out of the position that hurts the nerves without thinking about it. If we don't shift into a better position, the nerves eventually stop communicating with the brain - but that doesn't mean we stop feeling that limb. When signals go blank from a hand or a foot, the brain or the spinal cord can start sending signals just to fill in the blanks. Our hand isn't tingling. Our brain is. After that comes the numbness. This is what happens when we sleep deeply and wake up with a numb limb. When we move the limb again, the sharp stabbing pain we feel is all the nerves sending out signals as they come back online.
How Bad Can It Get?
Although we want to avoid any and all nerve damage, there's a difference between minor temporary damage and permanent damage. Pain and numbness is the best indication of the extent of the damage. Some pins and needles take a brusque shake and they're gone. Others have us hopping around for a while trying to shake our nerves back to working order. This is usually the worst of it, for normal sleep. It takes hours for real damage to happen, and any position that completely cuts off blood flow or pinches a nerve is too uncomfortable to sleep through.
Unless we are not in a state to respond to bodily signals. During surgery, when a drugged patient cannot wake up, doctors have to be careful to position the patient so as not to cut off blood flow. And sometimes patients drug themselves. There is a condition known officially as radial neuropathy. English and American doctors know it as Saturday night palsy, whereas French doctors, those romantics, know it as lover's arm. When a person is drunk, and falls asleep on a bench or a chair with the underside of the arm pressed to the edge of that chair, the radial nerve is compressed. (Alternately, if a French person doesn't want to wake someone whose head is resting on their inner arm, the nerve is also compressed.) Because a deeply drugged person is insensible to pain - or a person in love is willing to endure it - they don't move their arm, and they sleep for hours. When they wake up, their arms are numb for hours more. Most of the time, sensation comes back within a day. Sometimes the patient needs to be splinted, and the sensation doesn't come back for days. Sometimes the person requires surgery, or physical therapy. Very rarely, it gets even worse than that.
When an extremely unlucky person passes out for many hours, or passes out in an extremely stressed position, their cells literally die for lack of blood. One young woman passed out drunk with her legs folded under her - then was delayed in getting medical care - and lost both legs below the knee. One heroin addict passed out for over a day in a position that caused her permanent nerve damage and forced doctors to scrape necrotic tissue from her legs. People can lose function and lose limbs - but they generally have to be drugged or kept utterly still in order to do it. So be glad for just a little tingling.