Your body stays warm through a coordinated system of internal heat production, blood flow management, insulation, and hormonal control, all directed by a small region in your brain called the hypothalamus. At rest, your organs generate enough heat to maintain a core temperature around 98.6°F (37°C), and when that temperature drops, your body activates several backup systems to produce and conserve warmth.
The Brain’s Internal Thermostat
The hypothalamus acts as your body’s thermostat. It continuously monitors your current core temperature and compares it to the target of about 37°C. When your temperature dips too low, the hypothalamus triggers heat-generating and heat-conserving responses throughout the body. When you’re too warm, it does the opposite, increasing sweating and pushing blood toward the skin to release heat. This constant comparison and adjustment happens automatically, without any conscious effort on your part.
Where Your Body Heat Actually Comes From
Every cell in your body produces heat as a byproduct of metabolism, the chemical reactions that keep you alive. But some organs contribute far more than others. At rest, your brain accounts for about 24% of your body’s total heat production, your liver generates roughly 20%, and your skeletal muscles contribute around 18%. Together, those three sources produce nearly two-thirds of your resting body heat. Fat tissue, despite its reputation, generates less than 8% of resting heat.
This is why you feel cold when you haven’t eaten in a while. Your cells need fuel to run these metabolic reactions, and less fuel means less heat. In extreme cold, caloric demands rise sharply. Military studies found that soldiers in arctic conditions (around -28°C) burned roughly 4,200 to 5,000 calories per day, compared to about 3,100 calories per day in desert heat. Even relatively sedentary troops in mildly cold environments (around 0°C) burned about 3,400 calories daily. Your body simply needs more energy to maintain its temperature when the environment is working against you.
Shivering: Your Emergency Heater
When your core temperature starts to fall, the hypothalamus triggers shivering, rapid, involuntary muscle contractions that convert chemical energy into heat. Shivering is remarkably effective. During mild cold exposure, it can double your resting heat production. As your core temperature drops further toward 35°C (95°F), shivering can ramp up to roughly five times your resting metabolic rate.
That intensity comes at a cost. Shivering burns through energy stores quickly and is exhausting, which is why it works best as a short-term defense. If the cold exposure continues, your body needs other strategies to keep up.
Brown Fat: Heat Without Shivering
Your body has a second, quieter way of generating heat that doesn’t involve muscle contractions at all. Brown fat, a specialized type of fat tissue, exists specifically to produce warmth. Unlike regular white fat, which stores energy, brown fat cells are packed with tiny power plants called mitochondria. These cells contain a unique protein that essentially short-circuits the normal energy production process: instead of converting fuel into usable cellular energy, it diverts that energy directly into heat.
Babies have large deposits of brown fat because they can’t shiver effectively. Adults retain smaller amounts, typically around the neck, collarbones, and along the spine. When you’re exposed to cold, your nervous system activates these brown fat cells, and they begin burning fatty acids to generate warmth. This process, called non-shivering thermogenesis, can run quietly in the background without the exhaustion and energy drain of shivering.
How Your Body Traps Heat Inside
Producing heat is only half the equation. Your body also works hard to prevent heat from escaping, and it does this primarily by controlling blood flow. When you’re cold, blood vessels near your skin’s surface constrict, pulling warm blood away from the surface and redirecting it toward your vital organs. This response isn’t limited to your hands and feet. It’s widespread throughout your entire outer shell of skin, fat, and surface muscle.
The tradeoff is real: your core stays warm, but your skin, fingers, and toes get cold. The reduced blood flow lowers skin temperature, which is why your hands feel icy long before your core temperature actually drops. The layer of subcutaneous fat beneath your skin also plays a role. Fat conducts heat roughly half as well as muscle, making it a natural insulator that slows heat loss from deeper tissues.
Thyroid Hormones Set the Baseline
Your thyroid gland produces hormones that act like a dial controlling your baseline metabolic rate. The active form of thyroid hormone enters cells throughout the body and switches on genes that increase oxygen consumption, energy use, and heat production. It essentially tells cells to work harder and burn more fuel, which generates more warmth as a byproduct.
This is why people with an underactive thyroid often feel persistently cold. Their cells aren’t running hot enough. Conversely, an overactive thyroid can leave people feeling overheated and sweaty, because their metabolism is cranked up higher than normal. Thyroid hormones don’t respond to moment-by-moment temperature changes the way shivering does. Instead, they set the overall pace of your internal furnace.
What You Wear Matters More Than You Think
Your body’s internal systems can only do so much. Clothing works by exploiting a simple physical principle: air is a poor conductor of heat. Layered clothing traps thin pockets of warm air around your body, creating insulation that slows heat loss. A base layer pulls moisture away from your skin, because wet skin loses heat far faster than dry skin. A middle insulating layer (fleece, down, or wool) traps the most air. An outer shell blocks wind and rain from stripping away that warm air while still allowing moisture vapor to escape.
This layering system mirrors what your body does internally. Your skin and fat act as a natural base layer, vasoconstriction reduces heat flow to the surface, and clothing adds an external barrier. When any part of this chain breaks down, whether from wet clothing, wind exposure, or exhaustion that depletes your energy stores, your core temperature can begin to fall.
Why Some People Run Warmer Than Others
Normal body temperature isn’t a single fixed number. It ranges from about 97°F to 99°F (36.1°C to 37.2°C) across healthy individuals. Several factors influence where you fall in that range. People with more muscle mass tend to produce more resting heat. Those with higher thyroid activity run warmer. Body composition matters too: a thicker layer of subcutaneous fat provides better insulation, while a smaller body with more surface area relative to its volume loses heat faster.
Age plays a role as well. Older adults often have lower metabolic rates, less muscle mass, and reduced ability to constrict blood vessels efficiently, all of which make them more vulnerable to cold. Fitness level, hydration, recent food intake, and even sleep quality can shift your thermal regulation from hour to hour. Your body’s warming system is dynamic, constantly adjusting dozens of variables to keep your core temperature in that narrow, livable range.