What is shivering? Why our bodies shake when it’s cold.

You’re walking down a Chicago street on a blustery winter day, when a particularly strong wind almost whips you off of your feet. Despite your many layers, as well as a wool hat, gloves, and a scarf, you start shivering. Are you simply underdressed, or could this involuntary movement be the sign of something more serious? We talked to a couple of experts to find out the science behind why we shiver. 

What is shivering? 

“Shivering is a way for our bodies to generate heat when we are cold. This is done through muscle contractions,” says Dr. Natasha Bhuyan, a family physician based in Phoenix, Arizona. Shivering can significantly increase the body’s metabolism, which can tire you out more quickly. “It’s a really effective way to increase or maintain our core body temperature, but it does require a ton of energy.”

Like sneezing and goosebumps, shivering is involuntary. It’s a type of protective, physiological response to keep your body comfortable. According to Dr. Romina Sifuentes Palomino, a family medicine specialist with Keck Medicine of USC, the hypothalamus, which is a part of the brain that regulates bodily functions, can sense even a tiny drop in our body’s internal temperature. The hypothalamus “then triggers rapid muscle activity to help maintain stability.” 

Rest assured shivering is a healthy response to being cold, one that’s completely natural. “It’s kind of like a built-in thermostat response designed to prevent hypothermia,” a dangerous medical condition when the body’s temperature drops below 95 degrees Fahrenheit. 

Someone with severe hypothermia might show a lack of coordination or slurred speech. Video: How to spot and treat hypothermia / Canadian Red Cross / Croix-Rouge canadienne

At what temperature do our bodies start shivering? 

When it comes to the temperature at which our bodies start shivering, “it really kind of varies,” says Palomino. Overall, she says, most people will start to shiver when their core temperature drops below a normal body temperature. Normal body temperature varies from person to person, but on average it’s between 97 and 99 degrees Fahrenheit. 

“Factors such as age, body fat, medical conditions, and even your ability to acclimate to the cold can also shift that threshold,” says Palomino. For instance, a seven-year-old child might start shivering quickly when she’s cold because her body’s small size is less efficient at regulating temperature. 

The same goes for older adults, whose slower metabolisms, decreased blood circulation, and thinner skin make their bodies have to work harder to maintain their core temperature. 

When can shivering be a warning sign? 

While most of us shiver when we’re cold, that’s not the only reason you might shiver. Say you’re walking through a haunted house attraction and are about to turn a darkened corner, where you think there’s somebody waiting to jump out at you. Your body goes into fight-or-flight mode, a natural automatic response to danger that causes your adrenaline to surge. With your nervous system in high-gear your muscles may start contracting and suddenly you’re shivering. 

Feelings of anxiety or awe (like when you’re looking out over the Grand Canyon and marveling at its enormity) can cause a person to shiver. It’s also often a sign that you’re sick. “The interesting thing is that the same brain pathways that regulate temperature also respond to stress and illness,” says Palomino. So, for example, a fever-related shiver means that the body is trying to raise its set point—basically its internal thermostat setting—to fight off an infection.”

According to Palomino, when shivering is extreme or persistent despite warming up, especially among older adults, it may be a sign of something more serious. Look for other clues, such as fatigue, body aches, or fever, which can signal a medical issue like an infection. 

“If someone is feeling shaky, but they are not necessarily cold, this is definitely a reason to see a family physician to investigate,” says Bhuyan. 

Then there’s hypothermia, which is often caused by prolonged exposure to cold, wet, or windy conditions. “We shiver when water is being evaporated from our skin,” says Bhuyan. Since water conducts heat away from the body about 25 times faster than air, clothes that are sweaty or wet can significantly increase a person’s heat loss. 

When a person’s shivering is accompanied by signs of confusion and slurred speech, it’s important to get them dry and warm immediately. You can increase their body’s core temperature by layering on dry clothing (and removing anything wet) and giving them something warm to drink such as broth or hot tea. Gentle physical activity, like jumping jacks, can also help increase a body’s temperature. 

“In general shivering is protective,” says Palomino, “but it’s really important to understand the context around it. That’s what really matters.”

So if you find yourself outside and feel a slight shiver when a gust of cold air hits you, don’t worry too much. Instead, use it as an excuse to indulge in a hot chocolate and spend the afternoon warming by a fire. 

In Ask Us Anything, Popular Science answers your most outlandish, mind-burning questions, from the everyday things you’ve always wondered to the bizarre things you never thought to ask. Have something you’ve always wanted to know? Ask us.

 

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