Today I found out what causes limbs to “fall asleep”.

Technically known as “paresthesia”, this syndrome is caused by the compression of specific nerves. When you sit cross-legged, sleep with your arm above your head, or position any limb in such a way to put excess pressure on a nerve, that nerve will stop sending impulses normally. Should the pressure be great, or the duration be long, the nerve will eventually stop sending impulses altogether.  Any area the nerve services will essentially then “fall asleep”.  Think of a person standing on a garden hose. It’s hard for the water to get to the nozzle when the person’s feet get in the way. If the person’s heavy enough, or she stands on the hose for too long and the hose fully compresses, water will eventually stop flowing entirely. Once this pressure is relieved, your nerve will start to function normally again (hopefully) and you can now move your hand/legs/arms/feet. Nerves, like a well worn hose, may take some time to work properly (expand) and you may feel some tingling, “pins and needles”, during the process.

Nerve cells, for the most part, have their main bodies located in the spinal cord itself. They have what are known as “axons” that branch out to your limbs (and other parts of the body, but we will focus on the limbs) and carry nerve impulses out from the spinal cord. Together with another cellular protrusion known as a dendrite, these projections allow us to feel the world around us.

Nerve impulses require a healthy energy supply, known as the axonal transport system.  This well developed micro-vascular distribution method provides the blood flow needed to maintain the cells in good working order. If pressure is put on the correct spot, though, all the tiny arteries, veins and capillaries that supply the nerves’ nutrients become pinched off and the nerve cells begin to function abnormally. If you doubt me, hit the Ulnar nerve (the funny bone) on something and see how sudden, extreme pressure makes you scream!

Studies have shown that it doesn’t take much to cause the axonal transport system go haywire. External pressures of as little as 20 mmHg (about ½ pound per square inch) caused intermittent paresthesia due to the veins supplying the nerve having their blood flow reduced.  When you get to around 1 pound per square inch for around 2 minutes, this can cause numbness, impaired dexterity, and result in muscle weakness.

Compression isn’t the only thing that can cause limbs to fall asleep. Excessive vibration will also lead to paresthesia. Operating hand-held vibrating tools is an extremely common cause of this.  For example, dirt bike racers who need to grip handlebars tightly, while jumping frequently, experience this and commonly refer to these symptoms as “arm pump”. The process that causes these symptoms in the presence of vibration hasn’t yet been fully clarified, but these pulsations have been shown to injure the entire neuron.

Any discussion of what causes limbs to fall asleep would be remiss without talking about the common misconception that a limb falling asleep is caused by a lack of blood flow to the entire limb involved. While blood flow is blocked to the nerve specifically, if the entire limb were to be cut off from blood, severe life threatening problems would arise every time we sat “crisscross applesauce” in grade-school (see the problems with tourniquet use in the bonus factoids below).

In medical terms, this is referred to as compartment syndrome. When blood flow is halted or reduced, life sustaining PH balance is in jeopardy. The area affected begins to become extremely acidic and cells begin to break down. Waste products and lethal levels of electrolytes, like potassium, begin to build up. Once the pressure on the limb is released, this “acid blood” is then sent back to the heart with potentially lethal consequences. Ask all those people on death row if excessive potassium is a good thing.  They might want to reconsider investing in a portfolio that backs the production of potassium chloride.  Cardiac arrest anyone!

If you liked this article, you might also enjoy our new popular podcast, The BrainFood Show (iTunes, Spotify, Google Play Music, Feed), as well as:

• Hollywood Medical Myths Part 1: Shocking Someone Who Has “Flat-Lined” Can Get Their Heart Going Again
• Fact or Myth: Sodium Raises Blood Pressure
• How the Sun Burns Your Skin and How Sunscreen Prevents This
• When Doctors Literally “Blew Smoke Up Your Arse”
• Is Eating Your Boogers Good For You?

Bonus Facts:

• The three most common drugs used in execution by lethal injection are: Sodium Thiopental (causing anesthesia); Pancuronium Bromide (a paralyzer that causes you to stop breathing); and Potassium Chloride (causing cardiac arrest).
• The total cost for the drugs involved in lethal injection is about $86.08.
• In 2009, the only US supplier of Sodium Thiopental stopped production of the drug forcing several states to postpone some executions because they had to adjust their protocol for putting to death the accused.
• One of the most common diagnosis of chronic paresthesia is carpal tunnel syndrome.
• The use of tourniquets to stop blood loss in trauma patients was once thought to be a staple life-saving treatment. The resulting induced compartment syndrome, created by their use, has shown the risks of using them far exceed the benefits, in most cases. Once applied, a strict protocol of reperfusion (restoring blood flow) must be followed to ensure fatal metabolites will not be introduced into the bloodstream. After only 60 minutes of use of the tourniquet, the metabolites can cause damage to vital organs; after 2 hours, permanent injury to the limb affected will result; and after 6 hours of tourniquet application, amputation of the limb is the preferred treatment. So next time Timmy the neighbor skins his knee in a bicycle accident, make sure that you instill the nick-name “stubby” before you take off your belt and cinch it around his bloody leg.
• The use of tourniquets does have a place in certain situations, though.  For instance, the most common use of tourniquets today is in the military. The need for the rapid control of blood loss in a hostile environment has been shown to reduce battlefield deaths, despite the potential risk factor in using a tourniquet. Once the fight has ended, the need for such a method of bleeding control is then reconsidered.
• Carpal Tunnel Syndrome occurs more often in women than men and is most common between the ages of 30-60. It usually pops up in people who perform repetitive motion activities like typing, sewing, or playing musical instruments and gets its named from the “tunnel” that provides the pathway for the median nerve being narrowed. This creates a problem when repetitive activities cause minor swelling in the area surrounding the nerve. Pressure is then placed on the nerve and symptoms like numbness tingling and pain are the result.
• Prolonged numbness, tingling or loss of motor control, to any limb, should be evaluated by a medical professional. It could be a sign of a more serious problem like a tumor, peripheral neuropathy, or stroke.