Heat & Sauna - Heat & performance
Heat acclimation for exercise and hot weather
Last updated 29 September 2026. How we write this .
After repeated days of exercise in the heat you sweat more, your blood plasma expands and your heart rate runs lower. Much of this arrives within a week, more by two weeks, and it fades slowly once the heat stops.
Start here After repeated days of exercise in the heat you sweat more, your blood plasma expands and your heart rate runs lower. Much of this arrives within a week, more by two weeks, and it fades slowly once the heat stops.
By the end, you will understand Why is exercise so much harder in the heat? What changes in your body as you acclimatise? How quickly does heat acclimation happen? Key words used in this lesson Meta-analysis A statistical method for combining the results of multiple studies into a single estimate. More reliable than any one study, but bounded by the quality of what went into it. 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.
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Heat acclimation is what your body does when you exercise in hot conditions day after day. You start sweating sooner and more heavily, the watery part of your blood expands, and your heart rate and core temperature run lower at the same pace. Much of this arrives within about a week, more comes over the second week, and it fades gradually once you stop meeting the heat.
Scientists use acclimation for changes produced in a lab or heat chamber, and acclimatisation for changes produced by a real climate. The body responds in much the same way to both.
Why is exercise so much harder in the heat?
Working muscles give off a lot of heat. To get rid of it, your heart pumps warm blood to the skin, where sweat evaporates and cools you. In hot air the skin struggles to shed heat, and the heart has to supply the skin and the working muscles at the same time. As you sweat, blood volume shrinks, so each beat moves less blood and heart rate climbs.
The cost is large. In a 2015 study at the University of Zürich, Stefanie Keiser and colleagues tested seven fit men at 18°C and at 38°C. In the heat, their VO2max, the most oxygen the body can use in an all-out effort, was 7.2% lower, and their performance in a 30-minute time trial was 12.8% worse.
Outdoors the gap can be bigger still. Sebastien Racinais and colleagues took nine trained cyclists to a training camp in Qatar and had them ride a 43.4 km time trial outdoors at around 37°C on their first day. It took them 77 minutes, against 66 minutes for the same effort in cool weather at about 8°C. They set off at their usual power, then faded badly as they heated up.
What changes in your body as you acclimatise?
A 2015 review by Julien Périard, Racinais and Michael Sawka lists the main adaptations: better sweating, more blood flow to the skin, lower body temperatures, less strain on the heart and circulation, better fluid balance, changes in how fuel is used, and stronger protective responses inside cells. Three are measured most often.
Plasma volume. Plasma is the liquid part of blood. In Keiser's study, 10 days of training at 38°C raised it by 6%. More plasma lets the heart fill more fully with each beat and leaves more fluid to sweat with. Sweating. In the same study, sweat output went up by 26% and the sodium content of sweat fell by 19%. The body starts sweating earlier, sweats more, and holds on to more of its salt. Heart rate. At the same speed in the same heat, the heart beats more slowly. In a University of Brighton study, men's peak heart rate during a 30-minute run at 40°C fell by 14 beats a minute after five days of heat training. Core temperature follows. Acclimatised people start exercise slightly cooler and heat up more slowly, which buys time before the heat forces them to slow down.
How quickly does heat acclimation happen?
Faster than most training adaptations. A 2015 consensus statement from an international group of heat researchers, published in the British Journal of Sports Medicine, set out the timeline. Early changes appear within the first few days. Most of the fall in heart rate and temperature, and most of the rise in sweat rate, develop within the first week, then progress more slowly over the next two weeks. Near complete changes in the heart and in sweating take around 6 to 10 days, and the best endurance performance in the heat takes about two weeks.
The Qatar cyclists show the pattern in real conditions. Their hot time trial took 77 minutes on day one, 69 minutes on day six and 66 minutes on day fourteen. By the end of the camp they were as quick in the heat as they had been in the cool.
How much someone adapts depends on how hard, how long and how often they meet the heat, and on whether that heat is dry or humid, according to Périard's review. It may also differ between men and women. In the Brighton study, led by Jessica Mee, eight men and eight women trained in 40°C heat. After five days the men's core temperature and heart rate had fallen, but the women's had not, although the women were already sweating more. After ten days the women's core temperature had come down too. The authors concluded that the women needed the longer programme to reach the same stability.
How long does heat acclimation last?
It fades more slowly than it builds. In a 2018 meta-analysis of 12 studies, Hein Daanen, Racinais and Périard calculated that about 2.5% of the gains in heart rate and core temperature are lost for each day without heat. Sweat rate did not follow such a neat pattern, though longer acclimation periods seemed to make its gains last longer. The encouraging finding was about getting it back. Re-acclimation happened 8 to 12 times faster than the gains had been lost.
Does heat training make you faster in cool weather too?
This is where the research genuinely disagrees. In 2010 Santiago Lorenzo and colleagues at the University of Oregon put 12 trained cyclists through 10 days of exercise in 40°C heat, while 8 similar cyclists did the same training at 13°C. Tested in cool conditions afterwards, the heat group raised their VO2max by 5% and their time trial performance by 6%. Their plasma volume rose by 6.5% and their heart's maximum output by 9.1%. The cool group did not change.
Keiser's team tested the idea again five years later with a crossover design, in which each of the seven men did 10 days of training in the heat and, separately, 10 days in the cool. Heat training raised VO2max by 9.6% and time trial performance by 10.4%, but only when they were tested in the heat. At 18°C neither changed. In an extra test, the researchers expanded the men's blood volume by 538 ml with an albumin drip, a protein solution given into a vein, before a hot trial. It did not help them perform, and none of the adaptations the team measured lined up with the changes in performance.
So the gains in hot conditions are well established. The gains in cool conditions are not, and nobody is sure which adaptation does most of the work. Almost all of these studies involved small groups of fit young men, so the numbers may not hold for women, older adults or people who do not already train.
Does being acclimatised protect you in a heatwave?
It helps, but it does not make you heat-proof. The consensus statement notes that regular training in mild weather brings only partial acclimatisation, and it singles out the first hot days of the year as a risky time, before bodies have adjusted. Acclimatised people still get very hot. The Qatar cyclists finished every hot time trial with a rectal temperature of around 40.2°C, even after two weeks of acclimatisation.
The NHS says children, older people and people with long-term conditions such as diabetes or heart problems are more at risk of heat exhaustion and heatstroke. Heat exhaustion can bring dizziness, headache, cramps, heavy sweating and intense thirst. The person needs to be cooled down and given fluids, and you can contact NHS 111 if you are struggling to manage it. Call 999 if someone is still unwell after 30 minutes of cooling, or has hot skin that is not sweating, fast breathing, confusion, a fit or loss of consciousness. Those are signs of heatstroke, a medical emergency.
Acclimation can also be built without exercising in the heat, for example with a sauna after training. The lesson Sauna after exercise: adaptation vs extra fatigue covers those studies.
This lesson describes research on how the body adapts to heat. It is general education, not medical or training advice, so speak to your GP first if you have a heart condition or another long-term illness.
Related in this classroom Related in other classrooms The studies behind this page Adaptations and mechanisms of human heat acclimation: applications for competitive athletes and sports - Scandinavian Journal of Medicine and Science in Sports, 2015Consensus recommendations on training and competing in the heat - British Journal of Sports Medicine, 2015Heat training increases exercise capacity in hot but not in temperate conditions: a mechanistic counter-balanced cross-over study - American Journal of Physiology - Heart and Circulatory Physiology, 2015Effect of heat and heat acclimatization on cycling time trial performance and pacing - Medicine and Science in Sports and Exercise, 2015A comparison of males and females' temporal patterning to short- and long-term heat acclimation - Scandinavian Journal of Medicine and Science in Sports, 2015Heat acclimation decay and re-induction: a systematic review and meta-analysis - Sports Medicine, 2018Heat acclimation improves exercise performance - Journal of Applied Physiology, 2010Heat exhaustion and heatstroke - NHSSources for this classroom
Educational content only. Not medical advice, diagnosis or treatment. Always consult a qualified healthcare professional before changing your health regimen.