Cold Therapy - The basics
What cold exposure does to your body
Last updated 29 September 2026. How we write this .
Cold makes your body cut blood flow to the skin to hold heat in, then make extra heat by shivering and burning fuel. In one study, an hour in 14°C water raised metabolic rate by 350%.
Start here Cold makes your body cut blood flow to the skin to hold heat in, then make extra heat by shivering and burning fuel. In one study, an hour in 14°C water raised metabolic rate by 350%.
By the end, you will understand Why does cold water feel so much colder than cold air? What happens to your blood vessels? How does your body make extra heat? Key words used in this lesson Sequence The specific order of amino acids in a peptide. The sequence determines the shape the molecule takes and therefore what it can bind to, which is why changing one amino acid can change a compound entirely. Placebo An inactive comparison given so that the effect of expectation can be separated from the effect of the intervention itself. Without one, improvement over time cannot be attributed to the treatment. Full plain-English glossary
Where this information comes from Claim-cited evidence. Central research claims on this lesson are linked to specific studies or official guidance.
This lesson links the specific studies and guidance used for its central claims.
Our standard: expert or athlete opinion may illustrate a viewpoint, but it is never treated as proof. Read the editorial policy .
Report an error or outdated claim .
Cold sets your body two jobs at once: keep heat in and make more of it. The blood vessels in your skin narrow, your nervous system releases noradrenaline, and if that is not enough your muscles start to shiver. These defences give you time, but they have limits, and in cold water those limits arrive quickly.
The sudden gasp and racing heart of cold shock have their own lesson, Cold shock, blood pressure and who should be careful. This one follows what happens next.
Why does cold water feel so much colder than cold air?
Because water pulls heat out of you far faster. In their 2016 review of human cold adaptation, Hein Daanen and Wouter van Marken Lichtenbelt describe water as a good medium for cold exposure because it conducts heat much better than air and can absorb a lot of heat before it warms up. Standing in 10°C air is brisk. Sitting in 10°C water is a much bigger challenge.
Moving water strips heat faster still, because it keeps replacing the thin layer you have warmed next to your skin. Beat Knechtle's 2020 review of cold water swimming notes that heat loss from the skin is higher in wind and waves. Even shivering works against you in water. The movement stirs the water around you, and Daanen points out that the heat shivering makes is relatively minor compared with the heat lost in cold water.
Skin and core also cool at very different speeds. Skin temperature drops fast and drives what you feel. Core temperature, the temperature of your organs, changes slowly. Feeling frozen does not mean your core is cold, and feeling fine does not mean you are safe.
What happens to your blood vessels?
The first defence is to stop warm blood reaching the surface. The small vessels in your skin narrow, a response called vasoconstriction, on orders from a temperature-sensing region at the base of the brain. Less blood near the surface means less heat lost, and it is why skin goes pale and fingers and toes go numb first.
Hands and feet have a twist. After the first fall in temperature, blood flow to the fingers and toes can briefly surge back, then drop again, in waves. This is called cold-induced vasodilation. A 2022 review of cold habituation from the US Army Research Institute of Environmental Medicine, led by Beau Yurkevicius, says the evidence suggests it helps protect fingers and toes from local cold injury.
The signal behind much of this is chemical. In a 2000 study from Charles University in Prague, Šrámek and colleagues had young men sit in water up to the neck for an hour at three temperatures. At 14°C, blood levels of noradrenaline, a nerve messenger that tightens blood vessels, rose by 530% and dopamine by 250%. Adrenaline, the hormone most people link with a fright, did not change. Heart rate and blood pressure rose only slightly, by 5 to 8%.
The same study shows that water does things even when it is not cold. In comfortable 32°C water, heart rate fell by 15%, blood pressure fell by about 11% and urine output roughly doubled. The authors concluded that immersion works mainly through hormones, while the cold response runs through the sympathetic nervous system, the body's fight-or-flight wiring. So some of what people credit to the cold in an ice bath is really just being in water.
How does your body make extra heat?
If narrowing blood vessels is not enough, the body burns more fuel. Daanen's review explains that heat production rises by 10 to 30 watts in the first minutes of cold, before any muscle moves. This is non-shivering thermogenesis: heat made by metabolism without visible shaking. Then comes shivering, rapid involuntary muscle contractions, which can produce heat equal to about four times your resting metabolism. It cannot run for ever, because it uses up glycogen, the store of sugar in your muscles.
Šrámek's study shows how steep the rise gets. In 20°C water, metabolic rate went up by 93%. In 14°C water it went up by 350%, and core temperature still fell.
Part of the non-shivering heat comes from brown fat, a type of fat that burns energy to make heat instead of storing it. Until 2009, most scientists thought adults had too little to matter. That year van Marken Lichtenbelt's team in Maastricht scanned 24 healthy men in a 22°C room and again in a mildly cool 16°C room. Active brown fat showed up in 23 of the 24 during the cool session but not in the warm one. It was less active in men who were overweight or obese. The lesson Noradrenaline, brown fat and the cold stress response goes into this further.
What happens if you stay in too long?
Cooling works inwards in stages. Knechtle's review sets out the sequence for cold water. The first three minutes bring cold shock, the gasp, fast breathing and racing heart. After about three minutes, the nerves and muscles near the surface cool, so the arms and hands grow weak and clumsy and swimming gets harder. After about 30 minutes, hypothermia becomes the main threat. A further danger, collapse around the time of rescue, can strike just before, during or after someone is taken out of the water.
Hypothermia means a core temperature below 35°C, against a normal of around 37°C. The NHS lists the signs as shivering, pale and cold skin, slurred speech, slow breathing, and tiredness or confusion. It is a medical emergency: call 999 or go to A&E. While you wait, get the person somewhere sheltered, remove wet clothing and wrap them in a blanket. The NHS says not to use a hot bath, hot water bottle or heat lamp to warm them.
Does your body get used to the cold?
Partly, though not always in the way people hope. The change that appears first is habituation, a smaller response to something you have met before. Yurkevicius's review describes it as the earliest adaptation to short, mild exposures: skin stays warmer, shivering is weaker and the cold feels less unpleasant. Daanen notes it has been seen even after repeated cold showers.
Feeling better is not the same as being better protected. Daanen's review found that most studies of repeated cold water immersion reported a fall in metabolic rate of about 20%. In one, 14 men repeatedly sat in 14°C water until their core temperature fell to 35.5°C. Over 13 days, the time this took dropped from about 130 minutes to 80. They shivered less, so they cooled faster. Daanen concludes that big gains in heat production or insulation have appeared in populations over many generations rather than within one lifetime, although longer spells of milder cold can modestly raise non-shivering heat production. Cold acclimation: what changes over time covers this in detail.
Is any of this good for your health?
These responses are real and measurable. Whether they add up to better health is a separate question, and here the evidence is much thinner. The largest trial so far ran in the Netherlands in 2015. Geert Buijze and colleagues randomly assigned 3,018 adults with no cold-shower habit either to end their usual shower with cold water every day for a month or to carry on as normal. The cold-shower groups reported 29% less sickness absence from work, but they were ill on just as many days. Participants knew which group they were in, and the authors say a placebo effect cannot be ruled out.
Michael Tipton's group at the University of Portsmouth, reviewing the whole field in 2017, concluded that the evidence behind claims for cold water varies widely and that some still rests on anecdote and speculation. A surge of noradrenaline or a burst of brown fat activity is a short-term response. On its own it tells you nothing about your weight, your immunity or how long you will live. Most of these studies involved small groups of young, healthy men, so the findings may not apply to women, older people or anyone with a heart or circulation problem.
This lesson is general education about how the body responds to cold, not medical advice. If you have a heart condition, high blood pressure, Raynaud's or another long-term illness, speak to your GP before trying cold exposure.
Related in this classroom The studies behind this page Human whole body cold adaptation - Temperature, 2016Human cold habituation: physiology, timeline, and modifiers - Temperature, 2022Human physiological responses to immersion into water of different temperatures - European Journal of Applied Physiology, 2000Cold-activated brown adipose tissue in healthy men - New England Journal of Medicine, 2009Cold Water Swimming-Benefits and Risks: A Narrative Review - International Journal of Environmental Research and Public Health, 2020The Effect of Cold Showering on Health and Work: A Randomized Controlled Trial - PLoS One, 2016Cold water immersion: kill or cure? - Experimental Physiology, 2017Hypothermia - NHSSources for this classroom
Educational content only. Not medical advice, diagnosis or treatment. Always consult a qualified healthcare professional before changing your health regimen.