As spring emerges, a day at 50 degrees Fahrenheit (10 degrees Celsius) might prompt some to wear a light sweater, skip the jacket, or even opt for short sleeves, varying by individual preference. However, come autumn, that same temperature could have many of us digging out our heavy coats.

This isn’t merely a figment of your imagination. The perceived warmth in spring has both physiological and psychological explanations; following a harsh winter, your body undergoes adaptations that make 50 degrees feel quite comfortable.

“This is something I experience frequently with my research,” explains Cara Ocobock, an anthropologist at the University of Notre Dame. Ocobock, who researches human adaptation to cold environments, frequently travels to Finland to study reindeer herders who are regularly exposed to severe cold.

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“The human body is incredibly adept at acclimating to various temporary environmental conditions, and the adaptations it undergoes during these periods are also temporary,” Ocobock notes. One of these adaptations involves a heat-producing organ in adults, only identified recently.

Scientific American interviewed Ocobock to delve deeper into the bodily changes that occur during winter, including this unusual organ, and how these changes influence our reaction to the initial warmth of spring.

Have you personally felt this “50 degrees feels warm” phenomenon?

Indeed, I recall an instance from my last visit to Finland. I was 300 kilometers (about 185 miles) north of the Arctic Circle during what is typically the coldest part of the year. For about four or five days, temperatures didn’t rise above –20 degrees Fahrenheit (–29 degrees Celsius). However, within five days, temperatures soared to the positive 40s Fahrenheit (around five to 10 degrees Celsius), which is highly unusual for that location and time of year. After experiencing such extreme cold, my body began sweating at temperatures just above freezing; I didn’t even need a coat. It felt like my body was readjusting, cooling down from what it had grown accustomed to.

How quickly do these physiological changes occur when exposed to extreme temperatures?

While individual and population differences exist, the body begins to adapt quite swiftly. Changes can start within 24 hours and become more pronounced over seven to 10 days. These adaptations persist until you change environments again, at which point you lose your acclimatization. This applies to various conditions, including heat, cold, humidity, dryness, or high altitude. For instance, after returning from fieldwork in the Rocky Mountains to sea level, I could swim two full lengths of an Olympic pool without needing to breathe. Within two weeks, that capability had diminished.

So, how do our bodies adjust to cold weather?

Several systems in your body work together to manage these conditions. One immediate change is an increase in your resting metabolic rate—the amount of calories your body burns at rest—to generate more internal heat as you lose more heat to your surroundings.

Additionally, there are changes in how your blood vessels contract or expand in response to cold. In colder environments, vessels constrict to reduce blood flow and heat loss. Meanwhile, blood is directed towards deeper vessels that are less exposed to the cold air, while in warm conditions, the opposite happens.

We also observe an increase in the activity of brown adipose tissue—this is a hot topic in current research. Commonly referred to as “brown fat,” this type of fat burns to generate heat during cold exposure. Found in adults near the clavicles and along major deep blood vessels, this tissue is considered a separate organ because it uses energy specifically to produce heat, not for physical activities like running. Previously, it was thought that only infants had brown fat, which they would lose as they grew older. However, recent findings show that brown fat is present in adult humans across various populations.

What distinguishes brown fat from regular fat?

Brown adipose tissue is densely packed with mitochondria, which act more like furnaces than powerhouses in this context, bypassing the usual energy creation process to produce heat instead.

To date, we’ve identified brown fat in cold regions like Russia and Finland, as well as in temperate climates like Albany, New York, and even in tropical climates like Samoa. This suggests that brown adipose tissue may be an ancient evolutionary feature.

How does brown fat activity vary with the seasons?

A study by my former graduate student, Alexandra Niclou, examined seasonal changes in brown adipose tissue among residents in Albany. She discovered that during winter, individuals could maintain higher body temperatures via brown fat with less caloric expenditure, indicating that the tissue becomes more efficient with increased use for body heat regulation during colder months. This spring, I plan to further investigate these effects among reindeer herders and indoor workers in Finland.

Considering these factors, what likely occurs in our bodies on that initial warm day of spring?

During winter, an increase in resting metabolism and active brown fat helps keep you warm. When it suddenly warms to 50 degrees Fahrenheit, your body, still in high gear, produces more heat than usual, making you feel much warmer and prompting you to sweat. Adjusting to the new, warmer conditions typically takes a week or more.

There’s also a developmental aspect—where you grew up significantly influences how your body reacts to temperature extremes and seasonal changes. As a college professor in Indiana, I find it amusing to see students from different regions dress vastly differently in 50-degree weather, from shorts and T-shirts to heavy jackets, based on their upbringing. This also might influence how active and responsive their brown adipose tissue is into adulthood.

Do seasonal changes affect you if you spend most of the winter indoors?

Even if you spend a lot of time indoors during winter, these seasonal changes still affect you, though perhaps not as intensely. This is part of our ongoing research in Finland, comparing the adaptations of reindeer herders, who are often outside in extreme cold, to those of indoor office workers in the same region. However, since you still experience the cold outdoors, even briefly, your body responds.

Why is it crucial to understand our bodies’ acclimatization to extreme temperatures?

Understanding how our bodies rapidly adapt to temperature changes is increasingly important in the context of climate change, which is creating highly unpredictable weather patterns, such as ice storms in Texas. It’s vital to harness our knowledge of biology, behavior, and technology to help people acclimate because our bodies have physiological limits in handling extreme temperatures.