Sometimes I’ve woken up with my arm totally numb. Is this dangerous, or will I wake up before anything happens? I’m afraid loss of circulation will lead to gangrene.
When you wake up with a numb limb, you haven’t cut off the blood supply. You’ve just been putting pressure on a nerve.
The numbness in the arm is caused by pinching the nerve, not by loss of circulation. The circulation is just fine and can’t be cut off by sleeping on the arm.
I’m pretty sure that if you rest on the affected area long enough (say, several days or weeks), you will indeed cut off the blood supply, and severely or even fatally injure yourself. Isn’t that what a bedsore is? It’s not a concern for most people, but if you are immobilized in bed due to other injuries or in a coma, then you or whoever looks after you needs to make sure that you’re not putting pressure on any one part of your body for too long.
My understanding is this can happen if you get extremely drunk, normally we move in our sleep to avoid this issue, but it can get suppressed if intoxicated enough, and people have lost limbs as a result.
Otara
Wait wait, so if I say, sit in such a way that I’m sitting on top of one of my legs and it falls asleep that isn’t because I’m partially cutting off circulation?
If that’s true, consider my mind blown. I’d always thought that’s what was happening when a limb fell asleep.
I’ve also read that people born without the ability to feel pain tend to injure themselves while sleeping the same way.
Yup, it’s the nerves not the blood. I used to think it was a matter of bloodflow too, for years.
I knew a woman whose arm was partially paralyzed for months after she slept on it. It did eventually recover. But from reading that wiki page it looks like it was nerve damage. Maybe she fell asleep drunk.
The OP immediately reminded me of this little horror story I saw in the news a while back:
Woman Whose Legs Amputated After She Sat On Them Sues Hospital
OK, if it’s just pressure on the nerve, as opposed to circulation being cut off, what is the mechanism that limbs are lost?
Limbs are not literally lost. The function of the limb can be lost, but the limb doesn’t actually drop off.
Also adding the process in common usage to castrate bulls - apply a tight band around the testicles, and a similar process with dealing with hemorrhoids. Surely there is some way of cutting off circulation by pressure.that could lead to loss of a body part.
Through constrictive pressure, sure. Putting pressure on it from a single angle, as in sleeping on it, isn’t going to cause the limb to drop off under any normal conditions.
Oh wow, I didn’t know it was the nerve and not blood supply. Why doesn’t it happen immediately then?
About the rubber band castration, I’d heard of that too. I always wondered how do they get the band over the balls? If they were that tight, wouldn’t it be impossible to stretch them that much?
Wow, when I was younger and was fairly worried about this. I’ve always been prone to arms & legs falling asleep, and finding yourself with a big, cold lump of flesh that used to be my arm was always freaky.
Thanks Dope, I’ll sleep a teensy bit better now! 
I worked with a guy this happened to - he passed out drunk on New Year’s Eve sitting with his legs crossed, and when he finally came to, his legs were paralyzed. When I knew him about 9 months later, he’d had some surgeries, was partially recovered, but walked very hobbled and stiff-legged.
I thought it was loss of blood supply to the nerve. Is that not what causes troube when a nerve is pinched?
The Master speaketh:
http://www.straightdope.com/columns/read/76/is-it-dangerous-when-your-leg-falls-asleep
Rubber bands really do stretch just fine.
I don’t know about castration, but I did have something like that happen with a cat’s leg. Somebody wrapped rubber bands around our cat’s leg, tightly enough to constrict the blood flow. The cat didn’t come back to us for a couple of days. By the time it did, damage had already been done. The vet had to cut off the paw and all of the skin sloughed off up to the point where the rubber bands had been. It took years to heal to the point where we didn’t need to keep a bandage over the lower leg.
It’s true that the tingling you get in an extremity when you’ve slept on it wrong does not mean that the blood supply to the limb has been cut off. However, it is quite possible to damage a limb by ‘sleeping’ on it to such an extent that it causes a life-threatening problem that can require surgery and/or amputation - and not just because of loss of nerve function, but because of significant tissue destruction.
Normal sleepers on a nice soft bed move around enough to prevent compression damage to their tissues. However, people who are more deeply unconscious, otherwise immobile, and/or lying on hard surfaces can indeed cut off circulation locally. Skin damage can occur with as little as two hours of immobility at points of maximum pressure, leading to decubitus ulcers (bedsores). Deeper tissues can also suffer decreased blood supply because of local pressure. In a worst-case scenario, damage to muscles of an extremity due to prolonged compression leads to a cycle of swelling, worsening of blood flow, tissue damage, and more swelling. The result is ‘compartment syndrome’ as has already been mentioned. If the inelastic sheath that surrounds the muscle group is not cut open to relieve pressure, the damage can result in permanent deformity or even death.
Compartment syndrome can arise in persons who are immobile for multiple hours - a classic presentation is someone who sits on the toilet for an extended period either due to unconsciousness or simple inability to stand up. Patients in the OR also are susceptible, which is why so much care is put into making sure that extremities are padded properly.
So, while it’s true that you aren’t really cutting off blood flow to an entire arm or leg due to an awkward position, it is quite possible to compress muscles in such a way with simple body weight that an arm or leg has to be amputated or suffers permanent damage.