Your muscles are remarkably wasteful engines. During exercise, only about 20 to 30 percent of the energy your body burns actually moves your limbs; the remaining 70 to 80 percent escapes as heat. That enormous internal heat load is the baseline reason you get hot when you work out, and it explains why some people overheat more than others. Differences in body composition, fitness, hydration, hormones, medications, and even the weather all tweak how well your body sheds that heat, and when shedding falls behind production, your temperature climbs fast.
Your Muscles Are Built to Produce Heat
Every time a muscle fiber contracts, it splits a molecule of ATP for fuel. A fraction of that chemical energy becomes mechanical work, and the rest becomes thermal energy. Across most species and fiber types, the mechanical share is modest. In human skeletal muscle, overall efficiency sits in the range of roughly 20 to 25 percent, meaning three-quarters or more of every calorie you burn during a run or a bike ride turns straight into body heat.1PubMed. Advances in understanding the energetics of muscle contraction Even in the most efficient muscle ever studied (a tortoise’s, interestingly), the best-case conversion tops out around 47 percent. Human fast-twitch fibers land well below that, and mitochondrial losses on top of cross-bridge losses bring overall efficiency to roughly 15 to 35 percent depending on the species and fiber type.2Muscle and Exercise Physiology. Efficiency of Skeletal Muscle
The practical takeaway is simple: the harder you exercise, the more heat you dump into your own tissues. Your body has to move all that thermal energy from deep in your muscles out to the skin and then into the environment. When any link in that chain underperforms, heat accumulates and your core temperature rises beyond what feels comfortable or safe.
How Your Body Tries to Keep Up
Two main cooling mechanisms kick in during exercise. First, blood vessels in your skin dilate so that warm blood from your core flows closer to the surface, where heat can radiate and convect away. Second, your sweat glands ramp up, and evaporating sweat pulls a large amount of heat off your skin. During mild exercise, these two systems handle the load fairly well: the heart bumps its rate up a bit, skin blood flow rises, and sweat beads on the surface.3PubMed Central. Cardiovascular responses to hot skin at rest and during exercise
The trouble starts when exercise gets more intense or the environment gets hotter. At higher heart rates, the time your heart has to fill between beats shrinks. Sending extra blood to the skin on top of what your working muscles demand becomes a zero-sum game: stroke volume drops, and your cardiovascular system can no longer boost cardiac output to satisfy both cooling and exercise simultaneously.3PubMed Central. Cardiovascular responses to hot skin at rest and during exercise That cardiovascular bottleneck is one reason overheating hits hardest during hard efforts rather than easy jogs.
Body Fat and Body Size
People often blame extra body weight for overheating, and there is truth to that, though the mechanism is more nuanced than “fat insulates you.” When researchers compared groups matched for total body mass but with different body-fat percentages, the higher-fat group saw a greater rise in core temperature despite producing the same amount of metabolic heat and losing roughly the same amount of sweat. The likely explanation is that fatty tissue has a lower specific heat capacity than lean tissue, so it absorbs less heat per degree of temperature rise, and there may also be impaired sweat-gland signaling.4PubMed Central. A comparison of thermoregulatory responses to exercise between mass-matched groups with large differences in body fat
Body size itself cuts both ways. Larger people produce more total heat because they have more metabolically active tissue, but they also have greater absolute skin area to radiate from and a higher overall heat capacity, meaning it takes more energy to raise their temperature. Research in hot environments has shown that bigger individuals actually tend to show lower core-temperature rises than smaller ones exercising at similar intensities, even after accounting for fitness differences.5Journal of Thermal Biology. Human surface to mass ratio and body core temperature in exercise heat stress—a concept revisited So being small and lean does not automatically protect you from overheating, particularly in genuinely hot conditions.
Fitness and Heat Acclimation
One of the strongest predictors of how well you handle exercise heat is simply how fit and heat-adapted you are. A fitter cardiovascular system pumps more blood per beat, freeing up capacity to supply both the muscles and the skin. Beyond cardio fitness, spending time exercising in warm conditions triggers a suite of adaptations: your sweat glands become more responsive, you start sweating earlier, your blood plasma volume expands, your resting core temperature drops, and your cardiovascular strain eases.6PubMed. Adaptations and mechanisms of human heat acclimation: Applications for competitive athletes and sports
The plasma-volume expansion alone is striking. In one study, heat acclimation increased plasma volume by about 6.5 percent and boosted maximal cardiac output in both cool and hot conditions. VO2max increased by about 5 percent in cool conditions and about 8 percent in the heat.7PubMed Central. Heat acclimation improves exercise performance If you moved from an air-conditioned gym to outdoor summer running and felt terrible, a large part of the problem is that your body had not yet made these adjustments. Most acclimation protocols call for 10 to 14 days of progressively longer heat exposure to see the full benefit.
Dehydration Makes Everything Worse
Even modest fluid losses amplify overheating. Sweating is your most powerful cooling tool, but it only works if you have enough fluid volume to support it. As you become dehydrated, your body reduces blood flow to the skin and lowers whole-body sweat output, especially in hot environments.8PubMed. Effect of hypohydration on core temperature during exercise in temperate and hot environments Less sweat and less skin blood flow mean less heat leaving the body, so core temperature rises faster.9PubMed. Temperature regulation during exercise
If you routinely overheat during workouts, insufficient fluid intake is one of the easiest variables to fix. You do not need to follow an elaborate hydration protocol; drinking enough to keep your thirst satisfied and your urine a pale straw color covers most situations. The point is that starting a workout already mildly dehydrated, which is common if you exercise first thing in the morning or after caffeine, can set you up to overheat well before the exercise itself would push you there.
Humidity and Why Sweat Sometimes Does Nothing
Your sweat only cools you when it evaporates. In dry air, evaporation is efficient and your body can shed enormous amounts of heat. In humid air, the moisture gradient between your skin and the environment shrinks, so sweat drips off instead of evaporating and cooling stalls. What matters most is the absolute humidity, the actual water-vapor content of the air, rather than relative humidity, because absolute humidity directly determines how fast sweat can evaporate, and this becomes more important at higher work rates and higher air temperatures.10PubMed Central. Delineating the impacts of air temperature and humidity for endurance exercise
Research on runners exercising in progressively more humid conditions confirmed that as relative humidity climbed above about 60 percent, evaporative heat loss dropped significantly even though the runners kept sweating at the same rate. Core temperature rose steadily in the most humid conditions.11PubMed Central. The effects of a systematic increase in relative humidity on thermoregulatory and circulatory responses during prolonged running exercise in the heat This is why a 28°C day in a humid coastal city can feel far more oppressive during exercise than a 35°C day in a desert. The temperature alone is not the problem; it is the inability to evaporate sweat that traps heat inside you.
Medications That Quietly Raise Your Temperature
A surprisingly large number of common medications interfere with thermoregulation. A recent systematic review identified 110 drugs with high evidence for increasing heat vulnerability, another 390 with possible effects, spanning categories from cardiovascular drugs to psychiatric medications.12PubMed. Identification of potentially heat-vulnerability increasing drugs – a narrative review The mechanisms vary: some reduce sweating, some alter skin blood flow, others promote dehydration.
Among the best-studied offenders, drugs with strong anticholinergic properties raised core temperature by about 0.4°C under heat stress, largely by suppressing sweat output. Non-selective beta-blockers also elevated core temperature, though by a smaller margin. Interestingly, antidepressants and diuretics did not significantly alter core temperature in the trials analyzed, despite widespread assumptions that they would.13PubMed Central. The effect of prescription and over-the-counter medications on core temperature in adults during heat stress: a systematic review and meta-analysis If you take any medication and notice you overheat more easily than you used to, it is worth asking your prescriber whether the drug could be contributing. Antihistamines, bladder medications, and some antipsychotics are among the drugs with anticholinergic activity that people often do not realize could affect their heat tolerance.
Hormones, Age, and Sex Differences
If you menstruate, you may have noticed that exercise feels hotter during certain weeks. That is not in your head. During the mid-luteal phase, when progesterone and estrogen are both elevated, resting core temperature shifts upward by a fraction of a degree, and that elevation persists during exercise in the heat. In one controlled study, core temperature peaked about 0.2°C higher in the mid-luteal phase compared to the early follicular phase, despite similar perceived effort and thermal sensation.14PubMed. Menstrual cycle effects on thermoregulation while exercising in the heat The difference is small in absolute terms, but if you are already near your comfort limit, that extra fraction of a degree can tip you into feeling overheated.
Aging also erodes heat tolerance. As people get older, their sweat glands produce less sweat per gland, even though the number of active glands stays about the same. Research in older women showed that the reduced sweating was not simply a fitness issue; it appeared to reflect either diminished gland responsiveness or structural changes in the glands themselves.15PubMed. Effect of age on heat-activated sweat gland density and flow during exercise in dry heat This is part of why heat-related illness disproportionately affects older adults, and why an exercise routine that felt manageable at 40 may leave you feeling dangerously hot at 65, even at the same relative intensity.
Your Brain’s Emergency Brake
There is a reason overheating makes you feel terrible and saps your motivation to keep going: your brain is deliberately slowing you down. The hypothalamus, which acts as your body’s thermostat, monitors rising core temperature and progressively reduces your ability and desire to maintain exercise output. This manifests as the creeping fatigue, mental fog, and overwhelming urge to stop that many people experience during hard efforts in the heat.16PubMed. Exercise and heat stress: cerebral challenges and consequences
Researchers consider this central fatigue mechanism a safety brake against dangerous hyperthermia. The dopaminergic system plays a role: animal studies show that manipulating brain dopamine and norepinephrine levels in the hypothalamus changes how long and how hot subjects will exercise before stopping.17PubMed Central. Influence of brain catecholamines on the development of fatigue in exercising rats in the heat This is worth understanding because the sensation of overheating during exercise is not just your skin feeling warm. It is an active neurological signal that your brain generates to force you to produce less heat. Trying to “push through” that signal by sheer willpower can be genuinely dangerous in extreme heat, because you are overriding a mechanism that exists to prevent heat stroke.
Rare Genetic Vulnerabilities
For a small number of people, overheating during exercise is not just uncomfortable but physiologically abnormal, rooted in genetic variants that make their muscles release too much calcium in response to heat. The best-known example involves mutations in the RYR1 gene, which encodes the ryanodine receptor in muscle cells. Certain variants of this receptor are hypersensitive to temperature, triggering uncontrolled calcium release and runaway heat production, a phenomenon most associated with malignant hyperthermia during anesthesia but also linked to exertional heat illness in everyday exercise.18PubMed Central. Heat-hypersensitive mutants of ryanodine receptor type 1 revealed by microscopic heating
Genetic research into exertional heat illness has expanded the list of implicated genes well beyond RYR1. A recent study found variants across 14 genes in people who had experienced serious heat illness during exercise, including genes involved in mitochondrial function, cardiac muscle, and metabolic pathways.19PubMed Central. Genetics of Exertional Heat Illness: Revealing New Associations and Expanding Heterogeneity This does not mean every person who overheats easily has a genetic disorder. But if you have experienced exertional heat stroke, or if a close relative has had malignant hyperthermia during surgery, genetic testing may be worth discussing with a physician.
Practical Cooling That Actually Works
If you know you tend to overheat, pre-cooling before exercise is one of the most effective strategies available. A systematic review of endurance exercise in the heat found moderate evidence that cold-water immersion before exercise improved performance, and limited but promising evidence for ice slurry ingestion.20PubMed Central. Pre-cooling for endurance exercise performance in the heat: a systematic review In a head-to-head trial, runners who drank an ice slurry before exercising lasted almost as long as those who did a full cold-water immersion, and both groups significantly outlasted a control group that did neither.21PubMed. Pre-cooling with ice slurry ingestion leads to similar run times to exhaustion in the heat as cold water immersion Drinking a slushy before your run is far more practical than sitting in a cold tub, and the evidence suggests it works nearly as well.
Beyond pre-cooling, a few straightforward habits help:
- Start hydrated: Drink water in the hours before exercise, not just during it. By the time you feel thirsty mid-workout, you may already have enough fluid deficit to impair sweating.
- Exercise in the cooler parts of the day: Your resting core temperature follows a circadian rhythm, sitting lower in the morning and peaking in the late afternoon.22PubMed. Exercise and the circadian variation in body temperature measures Exercising when your baseline is already at its daily peak stacks the deck against you.
- Build heat tolerance gradually: Two weeks of progressively longer warm-weather workouts can trigger the acclimation adaptations described earlier, including earlier sweating, greater plasma volume, and lower resting temperature.
- Reduce intensity on hot days: The brain’s fatigue signal is not weakness. Dial back pace or load when conditions are oppressive, and save your hardest sessions for cooler weather or air-conditioned settings.
Does Workout Clothing Make a Difference?
Activewear marketing often implies that the right fabric will keep you meaningfully cooler. The evidence is less enthusiastic. A narrative review of sports clothing and thermoregulation concluded that few studies have found significant differences in core temperature, skin temperature, or performance between natural and synthetic fabrics, or between chemically treated and untreated versions of the same fabric.23PubMed Central. The Role of Sports Clothing in Thermoregulation, Comfort, and Performance During Exercise in the Heat: A Narrative Review An older review reached a similar conclusion: modest amounts of clothing and different fabric types did not meaningfully alter thermoregulation during exercise in warm conditions.24PubMed. Clothing and thermoregulation during exercise
That does not mean clothing is irrelevant. The amount of skin you expose matters more than the brand on the label. Looser fits allow more air circulation, and less coverage means more skin area available for evaporation. But if you have been searching for a magic moisture-wicking shirt to solve your overheating problem, the research suggests you are looking in the wrong place. Your physiology, fitness, hydration, and environment dwarf any effect your shirt fabric can deliver.
Why Humans Overheat at All
It is worth stepping back and appreciating that humans are, by mammalian standards, exceptionally good at dumping heat. Our nearly hairless skin and dense network of sweat glands evolved specifically for sustained activity in hot, open environments. Modeling work suggests that only when hair loss and sweating ability reached near-modern levels could our ancestors remain active during the hottest parts of the day in equatorial savannas.25PubMed Central. Avoidance of overheating and selection for both hair loss and bipedality in hominins We are essentially purpose-built for endurance exercise in heat, more so than almost any other large mammal.
The reason you still overheat is not that the system is broken. It is that muscles are thermodynamically inefficient across all animals, and even the best cooling system has limits. When you push your metabolic rate high enough, or when the environment blocks evaporation, or when dehydration or medication or body composition or poor acclimation handicaps any link in the chain, heat accumulates faster than it leaves. Understanding which of those links is weakest for you personally is the first step toward exercising more comfortably in the heat.