We have checked 54 claims about heat and cold.

“The first minute in cold water is the most dangerous part.”

Sudden immersion in cold water sets off a gasp and uncontrollable fast breathing that peak in the first minute, and breathing control usually returns after about a minute. Water-safety researchers attribute about 60% of deaths in the first minutes of immersion to this cold shock.

“Ice baths after lifting weights blunt your muscle gains.”

Cold water taken right after strength sessions for weeks led to smaller gains in muscle size and strength in several trials, and three meta-analyses agree on the direction. Most studies put only the legs into water at about 10°C for 10 to 20 minutes.

“Mixing alcohol and a sauna is dangerous.”

Half of the people who died in Finnish saunas in 1990–2002 had been drinking, and the share was higher in Swedish and Korean forensic series. In a small experiment, sauna plus alcohol dropped systolic blood pressure from 136 to 113 mmHg, where sauna alone left it at baseline.

“Regular sauna use lowers your sperm count.”

In a small Italian study, two 15-minute sessions a week at 80–90°C for three months cut sperm count and motility in 10 men. Every measured change had reversed six months after they stopped.

“Ice baths help you recover faster after exercise.”

Across dozens of small trials, cold water after hard exercise eased soreness and helped muscle power recover by the next day. The soreness benefit averaged about 0.3 points on a 0–10 scale in one meta-analysis, and the largest review found no faster recovery of strength.

“A cold plunge lifts your mood.”

People usually feel better right after a cold dip, and in the one randomized comparison of durations, five minutes lifted mood as much as twenty. None of the studies blinded people or used a placebo, so expectation can't be separated from the cold.

“Regular sauna use helps you live longer.”

One Finnish cohort ties frequent sauna use to fewer deaths. With longer follow-up the gap shrank from 40% to about 14%, and it disappeared in the fittest men and in women's heart deaths.

“Using a sauna four to seven times a week halves your risk of dying from heart disease.”

In one Finnish cohort, men who took four to seven saunas a week died of heart disease at about half the rate of men who went once a week. Later analyses of the same men found a smaller gap, and the link was missing in the fittest men and in women who went two or three times a week.

“Frequent sauna use cuts your stroke risk by 61%.”

The 61% comes from one Finnish cohort of 1,628 adults, where people who took four to seven saunas a week had fewer strokes than weekly bathers. The group sizes and stroke counts sit behind a paywall, and a Japanese study of daily hot baths found a smaller link.

“Sauna lowers your blood pressure.”

Blood pressure falls for a while after a sauna, and that short dip is what the popular numbers measure. Trials lasting weeks found little or no lasting drop from heat on its own, while one small trial found a clear drop when the sauna followed exercise.

“Pregnant women should stay out of saunas.”

The concern is a core temperature rising toward 39°C in early pregnancy, and hot tubs carry a clearer signal than saunas. For saunas the data are thin and mostly reassuring for 20 minutes at 70°C, and nobody has studied longer or hotter sessions.

“Sauna cuts your risk of dementia by 66%.”

The 66% rests on 8 cases of dementia among about 200 Finnish men who took four to seven saunas a week. A larger, independent Finnish cohort found no link in its most frequent bathers, and saunas above 100°C went with more dementia in its first 20 years.

“Infrared sauna therapy treats heart failure.”

In the largest randomized trial, two weeks of Waon therapy, which pairs a 60°C infrared sauna with rest under a warm blanket, improved walking distance in heart-failure patients. The trial missed its main goal of lowering the heart-strain marker BNP more than standard care, and nearly all the research comes from one Japanese network.

“Regular sauna use protects your lungs and prevents pneumonia.”

In one Finnish cohort, men who took four or more saunas a week were diagnosed with pneumonia and chronic lung disease less often. The link disappeared when the analysis accounted for men who died of other causes first, and the published abstract leaves that result out.

“Cold showers mean you get sick less.”

In a trial of 3,018 Dutch adults, people who ended their showers cold took 29% fewer sick days off work. Their days of illness did not differ significantly from those of people who showered as usual, and most people in every group missed no work at all.

“Sauna relieves chronic pain and arthritis.”

Small studies found pain eased with sauna in conditions from fibromyalgia to headache, and one randomized trial found headaches 1.27 points less intense. Most studies had no control group, and arthritis gains over four weeks were not statistically significant.

“A sauna after training boosts your endurance.”

Post-training saunas expand plasma volume and help athletes cope with heat, and six runners lasted 32% longer in a run to exhaustion. A randomized crossover trial of 13 men found no extra gain in hemoglobin mass or aerobic fitness over training alone.

“Saunas are dangerous if you have heart disease.”

People with stable coronary disease or heart failure tolerated sauna sessions in small studies, and experts reserve the warning for specific conditions such as unstable chest pain or a recent heart attack. The reported sauna deaths mostly involved existing heart disease, often with alcohol.

“Regular sauna use means you catch fewer colds.”

One 50-person trial from 1990 found fewer colds in regular sauna users, and in the last three of its six months they had about half as many. Nobody has repeated it, and the other support is a Finnish cohort that linked frequent sauna use to less pneumonia.

“Splashing cold water on your face resets your vagus nerve.”

Cold water on the face sets off the diving reflex, and the vagus nerve slows the heart within seconds. Measured vagal effects last up to about 15 minutes, and nothing in the research shows a lasting reset.

“The Wim Hof Method lets you control your immune system.”

Men trained in the method had a smaller inflammatory response to an injected bacterial toxin, and the breathing did most of the work. Cold training alone left that response unchanged in a later trial, and no study has followed practitioners through real infections.

“Sauna treats depression.”

One session of medical whole-body heating eased depression more than a sham session in a trial of 29 people. When a pilot trial added infrared-dome heat to talk therapy, it did no better than sham heat.

“A sauna before bed helps you sleep better.”

In an online survey, 83.5% of sauna users said sauna helped their sleep, and warm baths before bed improve sleep in measured studies. No study has recorded sleep after a sauna with lab equipment or a wrist tracker.

“Cold exposure improves your sleep.”

The overnight sleep data come from trained men after evening exercise, where one small study found fewer arousals after a cold bath and another found no change. No controlled trial has measured how a cold plunge affects sleep in people who aren't athletes.

“A sauna session raises your growth hormone 16-fold.”

The 16-fold rise came from ten men doing two hour-long saunas a day for a week, and it faded by day three. One ordinary session gives a small, brief bump in younger men and none in older men.

“Saunas protect women's hearts the way they protect men's.”

The one sauna study that counted heart deaths in women found no difference at two or three sessions a week, and its 73 most frequent female bathers had no deaths to compare. The benefit people quote was measured in men.

“Sauna makes your arteries more flexible.”

Arteries measured less stiff right after a 30-minute sauna in a study without a control group, and the change was already fading half an hour later. Randomized trials that ran for weeks, one of them in Finnish saunas, found no lasting change.

“A sauna session is as good as a workout.”

A hot sauna raises your heart rate about as much as moderate cycling, which is where the idea comes from. The heart speeds up without pumping more blood per beat, and no trial has shown that sauna alone builds fitness the way exercise does.

“Sauna helps you lose weight.”

The half kilogram or so you lose in a session is sweat, and it comes back once you drink and eat. In a randomized trial that measured fat, a 15-minute sauna after exercise added no fat loss beyond the exercise itself.

“Infrared heat penetrates 3 to 4 cm into your body.”

Sauna sellers were promising an inch and a half of penetration by 1998, with no study behind it. When researchers finally put probes in muscle, warming fell from 3.0°C at 1.4 cm to 1.1°C at 3.4 cm, and core temperature did not move.

“Cold exposure improves insulin sensitivity by 43%.”

The 43% came from eight men with type 2 diabetes who spent about six hours a day in 14–15°C air for ten days. When the same group repeated it at 16–17°C and kept people from shivering, insulin sensitivity didn't improve.

“Cold plunges reduce inflammation in your body.”

A 2025 meta-analysis found that inflammatory markers rose right after cold water, and both studies behind that result had people lift weights first. Studies of weeks of regular dips found cytokines unchanged or slightly higher.

“Sauna lowers inflammation in your body.”

Finnish men who took the most saunas had slightly lower levels of the inflammation marker CRP, and the link was weak. Trials that heated people for weeks found no change in CRP, and a single session nudges some inflammatory signals up.

“Alternating hot and cold works better than either one alone.”

After exercise, alternating hot and cold water eases soreness compared with resting and does no better than cold water on its own. In a sauna, cold dips between rounds made the heat easier to bear for one session, and nobody has tracked the long-term effects.

“Sauna helps you build and keep muscle.”

Heating one muscle for hours a day slowed its loss in immobilized legs and gave small strength gains in some trials. The one sauna trial that measured muscle growth found no extra gain from post-workout infrared sessions.

“A sauna session burns hundreds of calories.”

Measured by indirect calorimetry, an hour of passive heat in 40°C water burned 61 kcal more than rest, and an hour of moderate cycling burned 556 more. The big sauna calorie counts come from heart-rate watches, which read a heat-driven pulse as exercise.

“Sauna boosts testosterone.”

In most sauna studies testosterone did not change, and the null results include a week of two hour-long sessions a day and three months of regular use. Two studies found short rises right after the heat, and one of those rises came only when men were pushed to exhaustion.

“You can sweat out toxins in a sauna.”

Sweat carries traces of metals and chemicals, and the studies that report large amounts relied on sweat people scraped off their own skin. When researchers collected sweat carefully, a session's worth held a small fraction of what the kidneys clear in a day.

“Cold water treats depression and anxiety.”

No full-size randomized trial has tested cold water as a treatment for diagnosed depression or anxiety. In the closest one, cold showers with Wim Hof breathing helped women with high depressive symptoms no more than warm showers with slow breathing.

“Cold plunges burn fat by switching on brown fat.”

Cold does switch on brown fat in people. The one controlled study that measured fat used two hours of 17°C air a day for six weeks and took off 0.7 kg of fat, and nobody has measured fat loss from plunges.

“Sauna switches on heat shock proteins that slow aging.”

One sauna study found blood heat shock protein 72 up 48.7% on average after 30 minutes at 73°C, and a review of 51 controlled heating studies found no change overall. No study in people has tied heat shock proteins from a sauna to slower aging.

“A cold plunge raises your dopamine by 250%.”

The 250% figure is plasma dopamine after a full hour in 14°C water, and the same lab found no significant dopamine rise when it ran that hour four years earlier. Dopamine in the blood comes mostly from nerves and the gut, and it barely reaches the brain.

“Sauna improves your blood sugar control.”

The only sauna trial with a glucose test found blood sugar rose more after a session. Small hot-tub trials lowered fasting glucose, and a review of randomized heating trials found no effect on fasting glucose or HbA1c.

“Sauna boosts BDNF and grows your brain.”

No study has measured BDNF after a sauna, and we found none linking heat to brain growth. Hot water and a heating suit raised blood BDNF briefly in two small studies of men.

“Cold plunges boost testosterone.”

The studies that measured it found testosterone falling or holding steady after cold water. A few minutes in water below 4°C lowered total testosterone by about 19% in young men, and a 15°C soak after squats blunted the usual post-workout rise.

“Infrared saunas work better than traditional saunas.”

No trial has compared infrared and traditional saunas on any health outcome. In the one head-to-head physiology study, a typical infrared session left core temperature unchanged while a traditional sauna raised it 0.4°C.

“Sauna gives you clearer, younger-looking skin.”

No study has tested sauna against acne or wrinkles. One small study linked regular sauna use to a steadier skin barrier, and heat is a recognized trigger for rosacea flushing.

“Regular cold exposure helps you live longer.”

Nobody has followed people who use cold water to see how long they live. Reviews of winter swimming find physiological changes in small groups, and they can't tell whether swimmers were healthier to begin with.

“Eleven minutes of cold a week is the ideal dose.”

A 2021 paper added up the cold-water habits that seven Danish winter swimmers reported and got 11 minutes a week. The study never assigned or compared a dose, and its lab cold came from water-perfused blankets.

“You can sweat out a hangover in a sauna.”

The liver removes about 90% of alcohol, and sweat carries it at about the same concentration as blood, so a sauna barely touches the total. Finnish sauna researchers link the hangover phase to heart rhythm risk.

“End your sauna session on cold to burn more fat.”

No human study has compared finishing a sauna session on cold with finishing it on heat for fat burning or any other outcome. The rule carries Susanna Søberg's name, and her 2021 paper never mentions it.

“Sauna heat kills cancer cells.”

In cancer treatment, dedicated devices hold tumor tissue at 40–43°C alongside radiotherapy or chemotherapy. A sauna raises core temperature by well under 2°C, and the one cohort that tracked sauna habits against cancer found no link.

“Sauna activates the FOXO3 longevity gene.”

We found no study that measured FOXO3 in people after a sauna or any other heat treatment. The claim joins an untested gene effect to a longevity link from one Finnish cohort.

“Infrared sauna sweat is 20% toxins, where a normal sauna's is only 3%.”

No study backs these percentages. An infrared sauna seller was already crediting them to an unnamed American study in 2002, and measured sweat is at least 99% water.