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We’ve all been there. You awaken in the morning, and one of your hands is completely numb. It feels dead, heavy and simply won’t work. Perhaps there’s some tingling as well.

Or, you arise from a long dinner or movie and one of your legs feels that way. Then over a few minutes — maybe you shook your hands, stamped your foot — everything goes back to normal.

But did you ever wonder why in the world this happens? Read on!

Some nerves are unhappy

When someone complains to me about their hand or leg falling asleep. I usually explain, “The nerves are not happy.”

In general, numbness, tingling and other symptoms called paresthesia are most commonly due to abnormal nerve function. And when this is intermittent, temporary and related to holding one position for a long time, it’s rarely anything to worry about.

The cause in these cases is simply pressure on one or more nerves traveling into the hands or feet. When you remove the pressure (by changing position, for example), the problem goes away.

However, many other causes of nerve problems — more than 100, in fact — can cause similar, though more prolonged and persistent, symptoms, as noted below. If you have one of these conditions, you’re far from alone: An estimated 20 million people have a form of peripheral neuropathy that might make hands or feet numb or tingly.

A word on nerve terminology

So, what is peripheral neuropathy? It’s worth clarifying some commonly used medical terms.

Neuropathy means nerve disease.

Peripheral neuropathy is a condition affecting nerves in the peripheral nervous system, which includes nerves outside of the brain and spinal cord. Nerves of the legs and arms are part of the peripheral nervous system and tend to be the first ones affected by diseases of peripheral nerves.

Compression (or entrapment) neuropathy develops because of pressure on a nerve. Carpal tunnel syndrome, which occurs when a nerve becomes compressed in an already tight channel in the wrist, is one well-known example. Having your hand or foot fall asleep is another. Fortunately, the latter is quite temporary, while carpal tunnel syndrome is often chronic.

Paresthesia is a sensation of pins and needles, numbness or another abnormal sensation, often tied to peripheral neuropathy. Having your hand or foot fall asleep is a temporary paresthesia.

When to see your doctor

If your hands or feet fall asleep occasionally and normal sensation quickly returns, that’s fine. No need to call your doctor.

But call your doctor promptly if you have persistent numbness, tingling or other unusual sensations in your hands or feet. This is especially important if these sensations cause trouble with walking or holding onto things.

Your doctor should investigate further and will likely consider possible causes of peripheral neuropathy, including the following:

—Type 2 diabetes is the most common identifiable cause of peripheral neuropathy, accounting for nearly a third of cases. Sometimes it’s the first indication that a person has diabetes.

—Diseases of the liver, kidney and thyroid.

—Nutritional deficits, such as vitamin B12 deficiency or other vitamin deficiencies. Vitamin B6 is unique in this regard because too little or too much can cause neuropathy; too little is quite rare, but it’s possible to get excess B6 from supplements.

—Alcohol ingestion is probably the most common cause of toxic neuropathy. Alcoholics may also have nutritional deficiencies that can cause neuropathy.

—Certain medications, including some antibiotics and chemotherapy drugs, or lead, mercury, and other chemical and industrial toxins, may be culprits.

—Vascular disease, which occurs when the blood supply to a peripheral nerve is impaired, as with atherosclerosis. The nerve becomes unhealthy or dies.

Additional causes of neuropathy are infection, compression or trauma to nerves, and inflammatory or autoimmune conditions that affect nerves.

It’s worth noting that nearly a third of cases have no clear cause — a problem known as idiopathic neuropathy.

The bottom line: The causes of peripheral neuropathy are many, and range from the harmless and annoying to the intolerable and dangerous. When in doubt, see your doctor.

But try not to worry when your hands or feet fall asleep due to holding your arms or legs too long in one position, as long as the tingling resolves within minutes and doesn’t happen often.

Robert H. Shmerling, M.D., is a senior faculty editor at Harvard Health Publishing.

© 2020 by Harvard University