You could be pinching a nerve in your neck with a poor pillow. Your arms aren’t going to fall off, but you ARE asking for more pain than you are really required to suffer in life.
You would have woken up armless in a bathtub filled with ice with “CALL 911 OR YOU WILL DIE… OH WAIT, YOU HAVE NO ARMS, NEVER MIND” written on the bathroom mirror.
This reminds me of a bedtime incident I suffered about two weeks ago. Basically, I must have been sleeping on my arm, and when I changed position in my sleep my other hand fell and rested on my “dead” hand. Rudely shocked from my slumber as I felt “someone else’s hand” on my leg, I leapt dozily from the bed and proceeded to smack my head hard against the door. As I woke up to my senses there on the floor, I was left with a numb arm, a big red mark on my forehead, and a severely bruised pride.
I would suggest, therefore, that numb limbs can certainly cause you harm.
My sister, who is a consultant physician in a hospital, indicates that this is probably the correct diagnosis. In most situations of this kind your arm is not dying from lack of blood.
However, if someone has applied a tourniquet while you slept, or you are seriously into bondage, I suggest you loosen the ties until the feeling comes back. (In the case of bondage, beg your master/mistress to loosen the ties. A dead slave is not much use.)
Seriously, if the numbness does not go away, seek medical help soonest, as this can be a symptom of something serious like a stoke.
Slight hijack, but it always seemed to me like it would be fairly easy to really injure yourself when you’re in this kind of situation. For instance, your arm is numb, you roll off the bed, and your arm just crumples under you. Which seems like it would be quite bad.