Maybe you're shivering in your sheets, while your partner slumbers away, snug as a bug, right next to you. Or perhaps you have that one relative who's always wrapped in a blanket, even when the thermostat is cranked all the way up. You might simply feel like you can never get toasty enough, no matter how many layers you toss on.
Of course, it's a common enough phenomenon. But why do some people seem to be naturally colder than others? The answer may surprise you, and it has a lot more to do with metabolism and body type than you might think.
How Humans Generate Heat
Like all mammals, humans use chemical reactions in our bodies to keep our internal temperature toasty, and consistently so. If that temperature falls just a few degrees outside of our ideal norm, we face all manner of ills, ranging from mood swings to impaired immune function — and, in extreme cases, even death.
Sometimes, our bodies deliberately try to throw off this balance. The whole point of getting a fever is to slowly cook the sickness out of your body, killing off the pathogens in the process. Even a two-degree change can cause you to feel ill, and your inner microbes to perish, during a low-grade fever.
To create this life-giving heat, we have metabolism: A series of chemical reactions that takes place within the cells of living organisms. Think of metabolism like a car’s engine: It takes in fuel (food, in our case), burns it, and uses the resultant energy to produce motion and heat. While humans don’t literally “burn” our food to harness its energy, one of these metabolic processes involves a similar, less explosive reaction called respiration. And like an open flame, this, too, requires oxygen.
How Do Our Bodies Regulate Temperature?
It’s estimated that nearly 50% of the calories we consume just go toward maintaining our body temperature, according to a 2019 paper published in Current Biology. To ensure that this extra heat isn’t wasted, we also have numerous adaptations to make sure we don’t drop in temperature too quickly.
Ever wondered why you get goosebumps whenever it’s chilly? That’s a vestige of our furry mammalian ancestors, which puffed out their hair to trap heat closer to the skin. And shivering? That’s the equivalent to revving a car engine, vibrating our muscles so they give off more warmth. It’s this balance of heat generation and removal that dictates our core body temperature.
Our bodies also automatically constrict blood flow to extremities like the hands and feet when the cold hits, so we lose less heat to our surroundings. Since heat loss occurs at the skin-air interface, the more surface area you have exposed, the more you’re likely to lose your hard-earned body warmth to a chilly winter's day.
By the same token, those with less circulation to their limbs often have a harder time getting warm, since less heated blood from the rest of the body passes by.
How Does Cold Tolerance Vary Between Individuals?
This ratio of heat-generating volume to heat-losing surface area affects our propensity to get cold, as well. As such, a smaller body tends to lose more heat, since it has less volume to compensate for a comparatively higher area.
And the smaller you get, the harder it is to prevent this heat loss. Take shrews, for example. These tiny mammals, due to their diminutive stature, must kick their heat-generating metabolisms into overdrive, such that their hearts pump to the tune of over 800 beats per minute. The average human heartbeat seems sluggish by comparison, beating only 60 to 100 times per minute. The shrews must consume a hefty meal every few hours, lest the animals starve.
Far larger than shrews, human children, and especially babies, need extra bundling for this very same reason. A baby can lose heat four times as quickly as an adult, equating to a much higher risk of hypothermia.
The discrepancy can be affected by sex, as well. Women tend to be colder than men, on average, due to having generally smaller bodies. In a 2021 study published in Energy and Buildings, researchers found that most of the female participants surveyed were less comfortable in slightly chilly temperatures than the male participants, due to different metabolic rates and sweat responses. On the flipside, the men tended to sweat more in the heat.
Certain conditions, like hypothyroidism and anemia, can also affect the output of heat. Iron-deficient individuals, for example, are known to have difficulty regulating their internal temperature and generating enough body heat. That’s because hemoglobin, the protein in red blood cells that carries oxygen around the body to activate our inner furnace, requires iron in its construction.
Read More: 4 Unusual Illness Triggered By Cold Weather
Are There Any Benefits to Being Cold?
Still, the cold is not always something to fear. Researchers have found, for example, that cooler temperatures can help promote better sleep. People with insomnia, for example, typically have more trouble with thermoregulation — that is, staying cool when they’re trying to go to sleep. A chillier room, or a cooling cap, can also help one get more shuteye. (Scientists have long set the ideal sleep temperature at around 60 to 67 degrees Fahrenheit.)
Meanwhile, if you have weight loss goals, the feeling of coldness can push your body to start burning through so-called “brown fat” more vigorously. (Brown fat can be readily harvested for an energetic boost, whereas its counterpart, white fat, tends to accumulate more in our tissues.) Researchers found that lower temperatures prompted the body to convert more white adipose tissue into disposable brown fat for heat, according to a 2012 study in the Journal of Clinical Investigation.
Shivering can also play a role in the relationship between fat and heat. Per a 2014 paper published in Cell Metabolism, shivering in the cold can release hormones that instigate fat loss. In addition, the movement of the muscles burns calories, but scientists caution that it likely isn't a silver bullet for weight-related woes.
How Cold Can We Get?
Of course, moderation is key, as hypothermia presents a very real risk to those who have difficulty regulating their body temperature. Despite this, and the reality of our picky bodies, some exceptional individuals have made headlines defying these biological norms.
In 1980, on a frigid night in Minnesota, 19-year-old Jean Hilliard quite literally froze solid after being forced to emerge from her wrecked car following a crash. She was found just 15 feet from the home of Wally Nelson, a town resident, who took her to a nearby hospital expecting the worst.
Hilliard’s temperature didn’t even show up on the doctor’s thermometer. By a massive stroke of luck, she was able to be resuscitated without any permanent complications, perhaps due to her age and good circulation.
Some researchers are even trying to deliberately induce the chilling effects of being frozen solid. Cryogenic preservation aims to induce a state of complete stasis in the body by freezing it. Some of its proponents hope to one day use the technology to create human time capsules, able to be thawed decades later, hopefully when novel cures for clients’ ailments have been developed. For now, though, revival pods remain strictly in the realm of science fiction.
Our writers at Discovermagazine.com use peer-reviewed studies and high quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
Essays in biochemistry. Metabolism.
Journal of Anatomy. Anatomy of the heart with the highest heart rate.
Critical reviews in food science and nutrition. Iron and thermoregulation: a review.
The Journal of Clinical Investigation. Brown adipose tissue oxidative metabolism contributes to energy expenditure during acute cold exposure in humans.