You know that pins-and-needles feeling you get when you sit too long in the same position? There's a popular idea that it's because just because of bad blood flow, but it's actually not that simple.
What's causing that tingly feeling
That pins-and-needles effect is called paresthesia. It happens when your nerves are compressed, usually because you're putting too much pressure on them.
Nerves are what give you sensations, like feeling pain, through signals sent from your the rest of your body to your brain. If there's too much pressure on your nerves, they can't do their job of transmitting those messages.
Here's how it works: the pressure cuts down the blood flow in vessels that nourish your nerves. Without that blood supply, the nerves can't transmit messages from your body to the brain. They end up sending signals that the brain doesn't know quite what to do with, so it starts producing different sensations, like tingling or numbness.
When you relieve that pressure on the nerves, usually by switching your position, your nerves start to get their function back. For a little while, blood flows more freely to that part of your body, which makes the pins and needles worse for a bit. When the blood flow is restored and the nerves start working properly again, the feeling subsides.
How can you make pins-and-needles go away faster?
Some people say that shaking your sleeping limb or massaging it can help. Or — if it's your foot — walking on it. There isn't any real scientific or medical evidence to show that any of these strategies work (it doesn't seem to have been studied). However, they seem pretty safe to try. Just be careful if you're walking on an asleep foot — losing sensation can throw off your balance and make you fall.
Should you be concerned?
If you lose sensation in a limb frequently or if the effect lasts for longer than a few minutes, there might be some other, more concerning, cause.
Chronic parasthesis — a more serious version of pins-and-needles — can be a sign of diabetes. Two-thirds of diabetics develop some kind of nerve damage in their legs and feet because the disease weakens blood supply to nerves. Other chronic illnesses, like liver damage or kidney disease, can also cause the feeling. And in rare cases, a tumor pressing on nerves can have a similar effect.
More commonly, you might hear about carpal tunnel syndrome, which happens when nerves are pinched — often from repetitive stress like too much typing — and therefore can't communicate to the wrist, causing a perpetual feeling of numbness.
So if your pins and needles feeling happens often and not when you're putting too much pressure on a limb, see a doctor.
But if you have a body part fall asleep after sitting or sleeping in the same position for a long time, you're probably fine.