We've checked 55 claims about heat and cold.

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

Dropping suddenly into cold water makes you gasp and then breathe fast and hard in a way you can't control. That response peaks in the first minute, and breathing control usually comes back after about a minute. Water-safety researchers blame this cold shock for about 60% of deaths in the first minutes of immersion.

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

In several trials, people who got into cold water right after their strength sessions for weeks built less muscle and strength, and three pooled analyses point the same way. 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 death investigations from Sweden and Korea. In a small experiment, systolic blood pressure (the top number) held steady in a sauna alone and fell from 136 to 113 mmHg when alcohol was added.

“Regular sauna use lowers your sperm count.”

In a small Italian study, 10 men took two 15-minute sessions a week at 80–90°C for three months, and their sperm count and motility (how well sperm swim) dropped. Every change the researchers measured had reversed six months after the men 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 pooled analysis, and the largest review found strength came back no faster.

“A cold plunge lifts your mood.”

People usually feel better right after a cold dip, and in the one randomized trial that compared durations, five minutes lifted mood as much as twenty. The studies had no placebo and people knew what they were getting, so expectation can't be separated from the cold.

“Regular sauna use helps you live longer.”

One long-running Finnish study ties frequent sauna use to fewer deaths. The 2015 paper put the most frequent bathers' risk of death 40% lower, and later papers with longer follow-up and broader groups put it about 14% lower. The link didn't show up in the fitter half of the men or in the women the study added later.

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

In a Finnish study that tracked men for about 20 years, those who took four to seven saunas a week died of heart disease at about half the rate of once-a-week bathers. Later analyses of the same men found a smaller difference. The link didn't show up in the fittest men, and women who went two or three times a week did no better than women who went once.

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

The 61% comes from one Finnish study of 1,628 adults that found fewer strokes in people who took four to seven saunas a week than in weekly bathers. The group sizes and stroke counts are behind a paywall. 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 the popular numbers come from that short dip. Trials lasting weeks found little or no lasting drop from heat alone. The clearest drop came in one small trial where people used the sauna after exercise.

“Pregnant women should stay out of saunas.”

The worry is core temperature climbing toward 39°C in early pregnancy, and hot tubs show clearer signs of harm than saunas do. For saunas the data are thin and mostly reassuring for 20 minutes at 70°C, and longer or hotter sessions haven't been studied.

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

The 66% comes from 8 cases of dementia among about 200 Finnish men who took four to seven saunas a week. A larger Finnish study by a different team found no link in its most frequent bathers. In that study, saunas above 100°C went with more dementia during the first 20 years.

“Infrared sauna therapy treats heart failure.”

In the largest randomized trial, two weeks of Waon therapy (a 60°C infrared sauna followed by rest under a warm blanket) helped heart-failure patients walk farther. 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 study that followed men for about 25 years, those who took four or more saunas a week were diagnosed with pneumonia and chronic lung disease less often. The link disappeared once the analysis allowed 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 finished their showers cold took 29% fewer sick days off work. They didn't report significantly fewer days of illness, 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 blood plasma and help athletes cope with heat, and six runners lasted 32% longer in a run to exhaustion. When 13 men tried training both with and without sauna in a randomized trial, the sauna added nothing to hemoglobin mass or aerobic fitness.

“Saunas are dangerous if you have heart disease.”

People with stable coronary disease or heart failure tolerated sauna sessions in small studies. Experts save the warning for specific conditions such as unstable chest pain or a recent heart attack. Most of the reported sauna deaths involved people who already had heart disease, and alcohol often played a part.

“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. It stands alone, and the other support comes from a Finnish study 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. In the studies the vagal effects last up to about 15 minutes, so the research gives no sign of 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. In a later trial, cold training alone left that response unchanged, and no study has tracked whether practitioners get sick less.

“Sauna treats depression.”

In a trial of 29 people, one session in a medical heating device eased depression more than a sham session with the main lamps switched off. Real heat did no better than sham heat when a pilot paired talk therapy with an infrared dome.

“A sauna before bed helps you sleep better.”

In an online survey, 83.5% of sauna users said sauna helped their sleep, and measured sleep improves after a warm bath before bed. Sleep after a sauna has yet to be recorded with lab equipment or a wrist tracker.

“Cold exposure improves your sleep.”

The overnight sleep studies tested trained men after evening exercise. One small study found fewer brief awakenings 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 after the third day. 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 benefit people quote was measured in men. The one sauna study that counted heart deaths in women found no difference at two or three sessions a week. None of its 73 most frequent female bathers died of cardiovascular disease, so there was nothing to compare.

“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, and the idea grew from that. 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.”

When researchers put probes in muscle, the warming from an infrared cabin fell from 3.0°C at 1.4 cm to 1.1°C at 3.4 cm, and core temperature did not move. Sauna sellers were already promising an inch and a half of penetration in 1998, and no study backed the figure.

“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 pooled analysis of 11 studies found that inflammation markers rose right after cold water, and the two studies behind that result both had people lift weights first. When people took regular dips for weeks, blood markers of inflammation stayed flat or rose slightly.

“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 one 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 more than resting does, and cold water alone works at least as well. A 2026 sauna trial found that cold dips between rounds made the heat easier to bear.

“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 from people's breathing, an hour sitting in 40°C water burned 61 kcal more than rest, and an hour of moderate cycling burned 556 more. Heart-rate watches produce the big sauna calorie counts because they read a heat-driven pulse as exercise.

“Sauna boosts testosterone.”

Most sauna studies, including a week of two hour-long sessions a day and three months of regular use, found no change in testosterone. Two studies found short rises right after the heat, and one of those came only when men stayed in until they were exhausted.

“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, women with high depressive symptoms did no better with cold showers and Wim Hof breathing than with warm showers and slow breathing.

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

Cold does switch on brown fat in adults. In the one controlled trial that measured fat, men who spent two hours a day in 17°C air lost 0.7 kg of it over six weeks. Plunges haven't been tested for fat loss.

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

In one sauna study, blood levels of heat shock protein 72 rose 48.7% on average after 30 minutes at 73°C. A review of 51 controlled heating studies found no change overall, and the idea that these proteins slow aging comes from animal research.

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

The 250% came from blood tests after a full hour in 14°C water, and the same lab saw no significant rise when it ran that hour four years earlier. Most blood dopamine comes from nerves and the gut and 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 on HbA1c (a longer-term blood sugar measure).

“Sauna boosts BDNF and grows your brain.”

Two small studies of men found that hot water and a heated suit raised blood BDNF for a short while. The sauna itself hasn't been tested, and we found no study that tied heat to brain growth.

“Cold plunges boost testosterone.”

The studies that measured testosterone found it 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.”

There are no studies of how long cold-water users live. Reviews of winter swimming describe changes in the body among small groups of swimmers, and they can't tell whether the 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 in the lab it cooled people with blankets that had cold water running through them.

“A sauna sweats microplastics out of your body.”

No study has found microplastics in sweat or tested whether a sauna lowers the amount in blood or semen. The idea spread from Bryan Johnson's self-tests, and Johnson himself wrote that he can't prove the sauna caused his drop and that the particles are too big to sweat out.

“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 anything else. Andrew Huberman named the rule after Susanna Søberg, and her 2021 paper never mentions it.

“Sauna heat kills cancer cells.”

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

“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 ties that untested gene effect to a longevity link from one long-running Finnish study of men.

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

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