You sit too long with your legs crossed or lie with your arm under your head, and when you go to move the limb, it's tingling with a "pins and needles" sensation. But why?
We've got nerves running through our bodies that act as lines of communication between the brain and the other body parts, transmitting commands from the brain and relaying sensory information back to it for processing. With a sleeping limb, your nerves are going a little haywire because prolonged pressure has actually cut off communication between that limb and the brain. (The tingling sensation is technically called paresthesia.)
Pressure puts the squeeze on nerve pathways and blood vessels, so the nerves can't transmit signals properly, and the blood vessels can't bring oxygen and nutrients to the nerves. The cut-off interferes with the normal flow of information between the limb and the brain, and the signals going back and forth get jumbled. Some nerve cells stop sending info entirely, while others send impulses erratically.
The problem is compounded by the fact that our nerves are pretty specialized, and different kinds of nerves and sensory receptors receive different stimuli and transmit different information. (We've discussed another bodily oddity caused by this.) When the various signals get scrambled and aren't transmitted normally, the brain starts to misinterpret the info it's getting and generates an array of sensations, like warmth, numbness and that tingling feeling.
When a limb falls asleep, we usually try to "wake it up" and change positions. Blood flows back to the limb, giving a little boost to the misfiring nerves and making the tingling seem worse, but eventually the nerve signals begin to flow properly again. The pins and needles sensation is annoying for a few minutes, but it's a nice little prompt for us to relieve the pressure on a limb before serious nerve damage occurs.
This story originally appeared in 2009.