Why Do We Shiver? How the Body Makes Heat When It Gets Cold
We shiver because the brain makes muscles contract rapidly to generate heat when core temperature drops. See how shivering, brown fat and fever chills work.
We shiver because the brain orders skeletal muscles to contract rapidly and involuntarily, and those contractions turn chemical energy into heat. Shivering starts when the hypothalamus, the brain’s temperature control center, senses that the body’s core is cooling below its set point of roughly 98.6°F (37°C). It is one of the fastest ways the human body can raise its own heat output.
What controls body temperature?
The hypothalamus, a small region at the base of the brain, works like a thermostat. It receives signals from temperature sensors in the skin and from sensors that track the temperature of the blood and internal organs. When those readings drift away from the set point, it triggers responses to bring the core back into a narrow healthy range, usually between about 97°F and 99°F (36.1°C to 37.2°C) in a healthy adult.
Cold responses come in a rough order. The body first tries to keep the heat it already has. If that is not enough, it starts making more.
- Blood vessels in the skin narrow (vasoconstriction), so less warm blood reaches the surface where heat escapes. Fingers and toes get cold and pale first.
- Tiny muscles at the base of hairs contract and cause goosebumps.
- Brown fat begins burning fuel to release heat.
- Muscles start to shiver.
How does shivering actually make heat?
Every muscle contraction uses ATP, the molecule cells use to store energy, and no muscle turns that energy into motion perfectly. Most of the energy released is lost as heat. During normal movement that heat is a side effect. During shivering it is the whole purpose.
When you shiver, opposing muscle groups contract against each other many times per second. Because they pull in opposite directions, the body barely moves, so almost all of the energy ends up as heat in the muscle tissue. Blood carries that heat to the core. Shivering usually begins in the muscles of the trunk, neck and jaw, which is why teeth chatter, and can spread to the arms and legs as the cold continues.
Shivering is expensive. It burns stored fuel, mainly glucose and fat, much faster than resting metabolism does, and it is tiring to keep up for long. It also makes fine movements such as writing or using a phone clumsy.
What is brown fat, and how is it different from shivering?
Brown fat, or brown adipose tissue, makes heat without any muscle movement. This is called non-shivering thermogenesis. Brown fat cells are packed with mitochondria, which give the tissue its color. They contain a protein called UCP1 (uncoupling protein 1) that lets the mitochondria release energy from fuel directly as heat instead of storing it as ATP.
Newborn babies have a relatively large amount of brown fat, especially around the shoulders and upper back, and depend on it because they cannot shiver effectively. Adults have much less, mostly around the neck, collarbones and spine, but it can still switch on in cool conditions. Over days and weeks of cold exposure, the body can also adjust through hormones, including thyroid hormone, which raises overall metabolic rate.
Why do you shiver when you have a fever?
With a fever, the body is not cold. The thermostat has been turned up. Substances released by the immune system during an infection act on the hypothalamus and raise its set point. Your actual temperature is now below the new target, so the brain responds as if you were cold: skin vessels narrow, you feel chilly, and you shiver until your temperature climbs to the higher setting.
When the fever breaks, the set point falls back to normal. Now the body is warmer than its target, so it does the opposite: skin flushes and you sweat to shed heat. The pattern of chills followed by sweating is a direct result of the thermostat moving up and then down.
Why do people shiver when they are not cold?
Several situations can trigger shivering or trembling without a cold room:
- After general anesthesia, many patients shiver as the drugs that suppressed temperature control wear off and the body corrects a drop in core temperature that happened during surgery.
- Strong emotions such as fear or excitement release adrenaline, which can cause shaking that looks like shivering.
- Low blood sugar can cause trembling, sweating and a cold, clammy feeling.
- After a long workout in cool, wet or windy weather, sweat evaporating from the skin can chill the body quickly once activity stops.
Trembling that is persistent, comes with confusion, or happens with no clear reason is worth discussing with a doctor.
When is shivering a warning sign?
Shivering is a normal and useful response, but intense, uncontrollable shivering in the cold is an early sign of hypothermia, which medical sources define as a core body temperature below 95°F (35°C). Other early signs include slurred speech, clumsiness, confusion and drowsiness. MedlinePlus, from the US National Library of Medicine, lists these symptoms and the basic first steps.
A dangerous point comes when shivering stops while the person is still cold. In more severe hypothermia the body runs low on fuel and the nervous system slows down, so shivering fades even as temperature keeps falling. A person who was shivering hard and becomes still, confused or very sleepy needs emergency medical help. Wind and wet clothing pull heat away far faster than still, dry air, which is why the National Weather Service publishes wind chill warnings for cold weather.
Older adults, infants, people with some medical conditions and people who have been drinking alcohol are less able to sense cold or produce heat, so they can become hypothermic without much shivering at all.
Does eating help keep you warm?
Food supplies the fuel that both shivering and brown fat burn, and digesting a meal produces a modest amount of heat on its own. This effect is small compared with shivering, and a big meal mostly makes people feel warm and drowsy for other reasons. The way specific foods affect sleepiness is often exaggerated, as with the popular belief about turkey and tryptophan.
Quick answers
Why do teeth chatter when you are cold?
Teeth chatter because the jaw muscles are among the first to shiver. Rapid contractions of the muscles that open and close the jaw knock the teeth together.
Can you stop yourself from shivering?
You can briefly tense up and reduce it, but shivering is controlled by the brain automatically. The reliable way to stop it is to warm up: get out of wind and wet clothes, add dry layers and move to a warm place.
Why do goosebumps appear in the cold?
Goosebumps come from tiny muscles that raise each hair. In animals with thick fur this traps a layer of warm air. In humans, with little body hair, the effect on warmth is very small.
Do babies shiver?
Newborns shiver very little. They keep warm mainly through brown fat, which is why they lose heat quickly and need to be kept covered and warm.
