1. Shivering: How Muscles Make Heat When You Are Cold
When your body shivers on a cold morning, it is because your muscles are contracting in tiny, rapid movements. Normally, muscle contraction moves your arms or legs. However, during shivering, the muscles that bend a joint and the muscles that straighten it contract at the same time. This means the body does not move, but the energy used is converted almost entirely into heat [1]. In this way, shivering acts like a body heater that works without motion. It is effective, but because it uses muscles continuously, it is difficult to maintain for long periods.
There is a curious finding related to this. In experiments where people were exposed to cold little by little every day, some participants began to shiver less. This leads to a puzzle: if the "heater" is turned down, how does the body still cope with the cold?
2. A Canadian Experiment: Shivering Dropped 21% but Heat Production Stayed the Same
A research team from Canada asked nine adult men to participate in a four-week experiment. The men wore suits that circulated water at 10°C (50°F) for two hours a day, five days a week [3]. Before and after the experiment, the researchers measured the strength of shivering. They found that shivering intensity had decreased by about 21% in both the whole body and the thigh muscles [3]. However, this was a change in the strength of shivering, not necessarily a change in the total amount of heat produced.
When the researchers measured the total heat produced by the whole body in mild cold (about 18°C or 64°F), the amount before the experiment was 1623 kJ, and after the experiment, it was 1590 kJ [3]. The difference was only 33 kJ, or about 2%. So, while shivering dropped by about 20%, the total heat production remained almost unchanged. The research team interpreted this to mean that the job of producing heat had shifted from the muscles to another part of the body [3]. It is important to note that this was a small study with only nine participants, all of whom were men. Therefore, we cannot say for certain that this happens in all people [3].
3. Brown Fat and Non-Shivering Thermogenesis: Other Ways the Body Makes Heat
The candidate for this new heat source is "brown fat." Unlike white fat, which stores energy, brown fat is a special type of fat that converts energy directly into heat. It is considered the center of "non-shivering thermogenesis," which is heat production that does not involve shivering [2]. Shivering is seen as a short-term response to cold, while using brown fat is a strategy for dealing with cold that lasts longer [2].
In the four-week experiment mentioned earlier, the volume of active brown fat increased by 45%, and its oxidative metabolism increased by about 2.8 times (a 182% increase) [3]. A previous study by the same group with six men also showed that the volume of active brown fat increased from 66 mL to 95 mL [4]. Although brown fat is often thought of as something only babies have, adults have it too. In one study, 23 out of 24 men showed active brown fat when exposed to mild cold at 16°C (61°F) [7]. In a Japanese study at Hokkaido University, men who spent two hours a day in a 17°C (63°F) room for six weeks saw their heat production in cold increase from 108 kcal to 289 kcal per day [8].
Muscles are also thought to be a place where heat is produced without shivering. In mice, removing brown fat made it impossible to maintain body temperature during long-term cold exposure. While we cannot apply this directly to humans, it suggests that both brown fat and muscles are important [2]. But if the body's heat production changes, does the way we feel cold change at the same speed?
4. A Dutch Study: Cold Felt More Comfortable After 10 Days
A research team from the Netherlands had participants spend six hours a day in a room at 15–16°C (59–61°F) for 10 days [5]. During this time, the volume of brown fat increased from 665 cc to 913 cc, and non-shivering heat production rose from 10.8% to 17.8% of resting metabolism [5]. The participants' experiences matched these physical changes. After adapting, they felt the cold was warmer and more comfortable than before, and they rated their own shivering as lower [5]. However, the time it took to start shivering remained about 50 minutes, showing no change before or after [5].
"Getting used to cold" is not just one thing. A review of other studies shows that when people adapt to strong cold (like repeated immersion in cold water), discomfort decreases, but large changes in body heat production are rare; in some cases, metabolism even dropped by about 20% [6]. In contrast, when people are exposed to mild cold repeatedly, non-shivering heat production increases by 11% to 18% [6]. Getting used to how cold feels and increasing heat production are separate processes. The same phrase, "I am used to the cold," can mean different things depending on the context. This leads to another question: why do results differ between studies?
5. Why Studies Disagree About Whether Shivering Decreases
The same Canadian research group that did the first experiment had also run a similar study earlier with six men under the same conditions (10°C water, two hours a day, four weeks). In that study, the volume of active brown fat increased from 66 mL to 95 mL (a 45% increase), and oxidative metabolism in cold doubled (2.2 times). However, there was almost no difference in the strength of shivering before and after [4]. Brown fat increased in both studies, but shivering decreased in one and stayed the same in the other.
The materials do not explain why this happened, such as by pointing to differences in cold intensity or measurement methods. Because the number of participants was small and all were men, we do not know if the same thing happens in women, children, or older people. It is more accurate to say that "some studies show shivering decreases and brown fat increases, while other studies show shivering does not change" rather than stating it as a universal fact. So, how much can we really expect our bodies to adapt to cold?
6. How Humans Cope with Cold: What the Body Can and Cannot Change
A review paper on human adaptation to cold concludes that the parts of the human body that can be rebuilt to handle cold over a lifetime are limited. We mainly cope with cold through actions like wearing clothes and using heating [6]. This means that decreased shivering or increased brown fat are small internal adjustments. It is closer to the truth to say that these adjustments help only because we have clothes, heating, and food in our daily lives, rather than saying "my body becomes strong against cold."
You might wonder if increased brown fat could be used for dieting. In a study where people were exposed to 17°C (63°F) for two hours a day for six weeks, body fat decreased by about 5% (in 12 people in the cold group compared to 10 controls) [8]. However, the researchers themselves stated that it is difficult to incorporate cold exposure into daily life as a way to fight obesity [9].
Do not try to "practice" cold by enduring cold water or wearing thin clothes. This can be dangerous and lead to hypothermia.
7. How to Track Your Own Feeling of Cold in Daily Life
You do not need to expose yourself to cold to learn about this. Instead, you can record how cold you feel in your daily life. For example, when you step outside in the morning, rate the cold you feel in the first minute on a scale from "1: not cold at all" to "5: very cold." Do this for several days while wearing the same clothes. Note that a score alone cannot separate changes caused by temperature, wind, or clothing from changes in how you feel. If your score goes down, it does not mean your shivering or heat production has changed, because feeling used to cold is different from changing how your body makes heat. Conversely, even if your score does not change, small adjustments might still be happening inside your body.
The numbers in this article come from small studies. If you are interested in the data, you can look at the English papers listed in the sources, starting with the figures and tables, to see that "shivering strength" and "heat amount" are measured separately. These experiments were conducted and managed by professionals. Do not try to copy experiments like entering cold water or staying in thin clothes for long periods.
It is dangerous to try to get used to cold by enduring cold water or wearing thin clothes.