Why Do We Shiver When We're Cold?
― Muscles Make Heat Without Really Moving
Your teeth chatter and your shoulders tense and tremble in the cold. This isn't your body giving in to the chill. It is using your muscles as a heater, making heat in a hurry. And shivering has a clever trick that makes it better at producing heat than exercise.
It's an autumn evening. You're waiting outside to meet a friend, dressed lightly, and suddenly the wind turns cold.
Before you know it, your jaw is chattering in tiny movements, and your arms and back are trembling on their own. You tense up to stop it, but it won't stop.
You aren't moving yourself. So what is your body actually doing?
Shivering makes heat for just two reasons
When muscles burn fuel, only part of it turns into movement. Most of the rest becomes heat. Usually this is a flaw, but shivering turns it to good use.
In shivering, the muscles that bend and the muscles that straighten contract in quick alternation, and sometimes at the same time. Their forces cancel out, so no work is done on the outside world and almost all the energy stays in the body as heat.
Together, these two things make shivering a movement built for one job: turning fuel into heat on the spot. Let's take them one at a time.
Muscles turn most of their fuel into heat
Your body slowly burns sugar and fat from food inside your muscles to produce force. But this conversion is not very efficient.
When you walk or cycle, only about 20 to 30 percent of the fuel's energy becomes movement. The other 70 to 80 percent is heat. That is why exercise warms you up and makes you sweat.
Shivering makes this "heat-prone" trait its whole purpose. When you feel cold, the body's temperature control centre deep in the brain tells muscles all over the body to contract in tiny movements. It starts without any decision on your part, which is why it is hard to stop.
Figure 1 lets you see how much extra heat this makes. Try changing the strength of the shivering with the slider.
Your arm barely moves, so why does the heat increase?
In your arm, a muscle that bends the elbow and a muscle that straightens it sit on opposite sides. Normally one contracts while the other relaxes, and that moves the arm.
In shivering, both contract in tiny, rapid alternation, and sometimes together. It is like a tug-of-war where both teams pull equally hard: the rope barely moves. Look at the bottom of Figure 2.
Almost no "outside work" is done, such as lifting an object or carrying the body forward. So nearly all the energy from the burned fuel stays in the muscle as heat. The two bars at the top of Figure 2 compare where the energy goes when walking and when shivering.
Chattering teeth work the same way. The muscles that close your jaw tremble in tiny movements, and you hear that as teeth lightly knocking together.
Shivering makes heat, but it also uses fuel fast. If you shiver for a long time, the sugar stored in your muscles runs down, and the shivering can weaken. This is thought to be why you chill more easily when you are hungry and out in the cold.
Newborn babies are said to be poor at making heat by shivering. Instead, a special fat called "brown fat," found around the neck and back, makes heat directly without any movement. In recent years, researchers have found that a little of it remains in adults too.
Summary
Shivering in the cold is a movement that lets muscles make heat. Muscles naturally turn most of their fuel into heat, and in shivering, opposing muscles also pull against each other so that almost no outside work is done. As a result, nearly all the fuel energy becomes heat, and strong shivering is said to produce several times the heat of rest.
Shivering turns muscles built for moving
into a "stove that doesn't move."
Still, shivering is a sign that your body can still make heat. If shivering stops in the cold, that can be a danger sign. For more, read Why Does Hypothermia Happen Even on "Not-So-Cold" Days?, and for another response to the same cold, read Why Do We Get Goosebumps When It's Cold?
- Lightly clench one fist in front of your chest, and use your other hand to take note of how warm the inside of that wrist feels.
- Keep your elbow bent at 90 degrees, tense both the front and back of your arm, and "lock" it so it doesn't move, for about 30 seconds.
- Relax and touch your arm. Even though it hasn't moved, you may notice it feels slightly warm and a little tired.
Even when your arm isn't moving, the muscles are using fuel and giving off heat. Shivering is this same thing repeated in tiny, fast movements all over the body. Don't do this if you feel unwell or have any pain.
For those who want more ― terms, formulas and links to textbooksEach part is labelled with its level, from junior high science to university specialist courses
- Junior highCovered in junior high school science
- High schoolCovered in high school physics and biology
- High school+Advanced high school material, or textbook sidebars
- UniversityUniversity specialist courses (physiology, thermodynamics) not taught in high school
- ResearchTopics researchers are still investigating, not taught as settled even at university
Junior highTerms: this phenomenon has a name
- Shivering thermogenesis: in response to cold, skeletal muscles contract in tiny movements without conscious control, producing heat.
- Non-shivering thermogenesis: heat production without shivering, thought to be carried out by brown fat and similar tissue.
- Antagonist muscles: a pair of muscles that act in opposite directions, like the ones that bend and straighten the elbow.
Junior highHigh schoolChecking with a formula: how much can strong shivering warm the body?
Suppose a 60 kg person shivers for 10 minutes at 5 times the resting rate. We calculate how much body temperature would rise, assuming no heat escapes through the skin. Heat is in joules, and the rate of heat production is in watts (joules per second).
| In symbols | ΔT ＝ ( Pshiver − Prest ) × t ÷ ( m × c ) |
| In words | Temperature rise ＝ extra heat rate × time ÷ (body mass × specific heat) |
| Where it comes from | From conservation of energy (the first law of thermodynamics) and the definition of specific heat (the heat needed to raise something by 1 ℃). In shivering almost no outside work is done, so the fuel energy builds up in the body as heat. |
| Symbol | Meaning and unit |
|---|---|
| Prest | Rate of heat the body makes at rest (watts) |
| Pshiver | Rate of heat the body makes while shivering (watts) |
| t | Time spent shivering (seconds) |
| m | Body mass (kilograms) |
| c | Specific heat of the body (heat to raise 1 kilogram by 1 ℃, in kilojoules) |
| Adult basal metabolism (per day) | Said to be about 1500 kilocalories |
| 1 kilocalorie | 4.184 kilojoules |
| Heat production in strong shivering | Said to be up to about 5 times the resting rate |
| Specific heat of the human body | Said to be about 3.5 kilojoules (heat to raise 1 kg by 1 ℃) |
| Daily basal metabolism in kilojoules | 1500 × 4.184 ＝ 6276 |
| Per second (1 day ＝ 86400 seconds, in joules) | 6276000 ÷ 86400 ≒ 73 |
| Heat in strong shivering (watts) | 73 × 5 ＝ 365 |
| Extra heat compared with rest (watts) | 365 − 73 ＝ 292 |
| Heat built up in 10 minutes (600 seconds) (joules) | 292 × 600 ＝ 175200 |
| Heat to raise the whole body by 1 ℃ (kilojoules) | 60 × 3.5 ＝ 210 |
| Temperature rise (℃) | 175.2 ÷ 210 ≒ 0.83 |
If no heat escaped, 10 minutes of strong shivering would raise body temperature by nearly 1 ℃. In reality, cold wind keeps taking heat away, so body temperature doesn't rise; shivering keeps you warm by making up for what is lost.
| Share that would become outside work when walking on the same fuel (watts) | 365 × 0.25 ≒ 91 |
When you walk, about 91 watts of this goes into the work of moving your body and does not become heat. Shivering can turn that share into heat too, so it is an efficient way to warm the body with the same fuel.
High schoolHigh school+Temperature control and muscle energy
High schoolBody temperature is kept steady by the temperature control centre in the hypothalamus of the brain. When it receives signals from cold receptors in the skin and from changes in blood temperature, it narrows the blood vessels in the skin so less heat escapes, and if that is not enough, it makes heat by shivering. The energy currency inside cells that muscles use to contract is ATP.
High school+When a muscle breaks down ATP to contract, the share that becomes outside work (mechanical efficiency) is said to be about 20 to 30 percent in activities such as cycling. In shivering, antagonist muscles work at the same time, so the outside work cancels out. Efficiency is close to zero, which means almost all the energy becomes heat.
UniversityShivering and non-shivering thermogenesis
In physiology, heat production in response to cold is divided into "shivering thermogenesis" and "non-shivering thermogenesis." The main example of the latter is brown adipose tissue, where the mitochondrial uncoupling protein (UCP1) makes fuel energy escape directly as heat instead of being turned into ATP. The heat balance follows the first law of thermodynamics, and changes in body temperature can be estimated with the specific heat capacity. Human shivering is known to be recorded as a continuous series of tiny contractions with waves of stronger and weaker activity, every few seconds, laid over it.
📖 Derivations of the formula and further reading: Shivering (Wikipedia, Japanese) ／ Brown adipose tissue (Wikipedia, Japanese)
ResearchWhat is still not clear
- How much does brown fat work in adults? Imaging has confirmed that adults have brown fat, but the amount and activity vary greatly between people, and research continues into how much it matters for controlling body temperature and weight.
- How does shivering change as we get used to the cold? There are reports that people living in cold environments shiver less and make more heat without shivering, but the mechanism and the differences between individuals are still debated.
- Can muscles make heat without shivering? It is thought there may be a pathway inside muscle that makes heat without contraction, and researchers are studying how useful it is in humans.
In other words, this article is also "an explanation of what we know so far." Even an everyday reaction like shivering still leaves questions without answers.
Links to textbooks (by level)
| Level | Subject and unit | Where in this article |
|---|---|---|
| Junior high | Science: "Structure and function of animal bodies," "Energy" | How muscles and bones work, conversion to heat |
| High school | Biology: "Maintaining the internal environment"; Physics: "Heat and energy" | The temperature control centre, calculation using specific heat |
| High school+ | Biology: "Muscle contraction and ATP" | Mechanical efficiency of muscle |
| University | Physiology, thermodynamics | Shivering and non-shivering thermogenesis, first law of thermodynamics |
| Research | Metabolism and energy physiology | Brown fat in adults, adapting to cold |
| ― | Links to daily life | Why you chill more easily when hungry and out in the cold, what it means when shivering stops |
- Shivering - Wikipedia (シバリング)
- Brown adipose tissue - Wikipedia (褐色脂肪組織)
- Basal metabolism - Wikipedia (基礎代謝)
- First law of thermodynamics - Wikipedia (熱力学第一法則)
- Haman, F. "Shivering in the cold: from mechanisms of fuel selection to survival." Journal of Applied Physiology, 2006.
- Irisawa, Masami (ed.), Textbook of Thermal Physiology (『体温生理学テキスト』), Bunkodo (文光堂)
※This article is a general-audience science explainer. The figures given are rough guides to help you understand the mechanism. If you keep feeling cold or shivering, or if you feel foggy or confused, don't hesitate to consult a medical professional.