|MadSci Network: Neuroscience
I've prepared an answer which goes into this issue in some detail. If you've got things to do, people to see, just read the summary at the end. If you'd like to find out what can happen to intoxicated people who sleep in strange positions, read the whole thing. I have placed links to other sites which will provide more information for some terms. Some of these sites are bibliographic, so if you spend some time you should should be able to do further searches on that topic if you wish.
We've all had this experience before: You're sleeping peacefully, and then you begin to wake up with the impression that something is wrong. The next thing you know, you're sitting upright in the bed, vigorously massaging your arm trying to get the feeling back. For a brief instant you panic, and the horrible thought crosses your mind that the feeling is never going to return. Meanwhile, your arm is flopping around like it doesn't belong to you, like its not even a part of you. Then, thankfully, you feel some tingling, and soon the feeling returns. You flex your arm a couple times just to make sure everything is OK and settle back to sleep, making sure your arm is beside you, not beneath you.
What I've just described is a classic case of mild, temporary nerve compression which causes your arm to "go to sleep". When a nerve is compressed, eventually the feeling of the body part supplied by that nerve goes away. However, before this happens, we often feel a tingling. Even more noticable, when the pressure is released the body part may feel hot, have a disagreeable tingling or pricking sensation, or may even feel cramped. These sensations are called paraesthesias. A paraesthesia is a non-painful sensory phenomenon which occurs in the absence of natural stimulation of sensory receptors.
It turns out that the numbness, tingling and paralysis caused by mild, temporary nerve compression is not due to the compression of the nerve per se, but is rather due to cutting off the blood supply to the nerve. Cutting off the blood supply to an area of the body is called ischemia. Ischemia is about the worst thing that can happen to a body part. Since that part recieves no new blood, it doesn't get oxygen, it doesn't get food (glucose) and it doesn't get its wastes (carbon dioxide, lactic acid, whatever) carried away. Nerves are particularly succeptible to ischemia because it takes a lot of metabolic energy to transmit nerve impulses.
Fortunately, the circulatory system of a nerve is specially designed to protect it from reduced blood supply, say from a blocked artery. Unlike most tissue, every nerve actually has 2 interconnected systems which deliver blood to it. This means that if the circulation to a body part (for example your arm) is greatly reduced, the nerves will still get enough blood to adequately function. The other specialization nerves have is that they normally receive much more blood than they need to function. This large safety zone means that even if circulation is impaired, the nerve will probably get enough blood to work well. In critical situations, a nerve can even work for some time by absorbing oxygen and glucose from surrounding muscles!
All of these back up systems mean nothing, however, if the nerve itself is compressed. A nerve is made up of the axons of neurons and connective tissue. The blood vessels which supply the nerve run in the connective tissue that surrounds the axons. So, these blood vessels are actually in the nerve. Thus, if the nerve is compressed, the blood vessels are also compressed, and blood supply to the axons is cut off. Ischemia sets in.
A series of very thorough experiments by Merrington and Nathan in the late 1940's showed that it is the lack of blood supply, and not the pressure on the nerve itself, that causes the sensory and motor symptoms. They also showed that the sensory effects that occur when compression is released are not due to blood reentering the body part, or due to activation of sensory receptors in the body part. The paraesthesias originate in the nerve itself. This is perhaps most dramatically shown when a nerve is compressed in a person with an amputated arm. They report tingling in their nonexistent fingers and hand!
The reason that ischemia causes paraesthesia is that eliminating a nerve's blood supply changes the excitability of the nerve. Initially, the nerve briefly becomes superexcitable. This produces the initial tingling. As the ischemia contines, the nerve becomes steadily less excitable, and after several minutes conduction of nerve impulses stops completely. Experiments have shown that your skin feels numb about 13-15 minutes after the blood flow is arrested, and that all sensation, except for pain, is gone after about 18 minutes. Muscular weakness begins at about 25 minutes, and paralysis sets in after 30 minutes. If you wake up with an "asleep" arm that you are unable to move, that means the circulation has been cut off for at least 30 minutes.
When the blood supply is restored, feeling begins to return in about 30 seconds to one minute. Many people report a period if intense tingling that can last up to 5 minutes. This "tingling" or "prickling" feeling increases if you stimulate the area (Remember how it feels if you stomp a "sleepy" foot to help it wake up?) Some people also get a cramping sensation. These feelings come about because the nerve becomes superexcitable when the blood supply is returned to normal.
If the circulation is cut off for more than about 40 minutes, the effects can be much more serious than when your arm "falls asleep". At 40 minutes, it can take several hours until normal feeling and use of the arm is restored. If the compression lasts for hours, it may take several weeks for a full recovery. These longer lasting nerve compressions are common enough that doctors have given some of them specific names. For example, Lover's Palsy occurs when the head of a sleeping person puts pressure on the nerves which travel through the arm pit. Saturday Night Palsy is a compression of the radial nerve which occurs when intoxicated people sleep with their arm draped over a chair or with someone sleeping on their arm. Severe cases cause complete wrist and finger extensor paralysis and require a wrist splint to prevent the hand from taking on a claw form, and can take several weeks for full recovery. Perhaps the most well known nerve compression disorder is Carpal Tunnel Syndrome, which is a compression of the median nerve of the arm. It produces numbing, tingling, and burning in the fingers and hand. It also reduces the ability move move your thumb to your fingers, which can be a major problem.
Mild nerve compression causes the familar experience of an arm or leg "falling asleep". Your arm falls asleep not because of the pressure on the nerve, but because the pressure deprives the nerve of its blood supply, a condition called ischemia. Ischemia causes changes in the excitability of nerve fibers. Initially, the nerve becomes superexcitable, and so as your arm or leg "falls asleep" you might notice a slight tingling or pickling sensation as the nerve spontaneously fires. As the ischemia continues, the nerve steadily becomes less excitable, until finally it is unable to fire at all. Once normal blood flow is restored, the nerve enters a prolonged period of superexcitability, which causes a more pronounced and disagreeable tingling, and sometimes even a cramping sensation. These sensations originate from the nerve itself and not the sensory endings; amputees can sometimes experience their non-existent limb "falling asleep"!
There are other issues which could be discussed regarding nerve compression, such as the biophysical basis of paraesthesia, but I have omitted them in the interests of space. If you have any questions or comments, or would like a list of the references I used to construct this answer, please send me an email.
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