The spike is documented. What happens after it is the part nobody checked.
The researchers who read all forty studies couldn't recommend which sauna to use for any specific health goal.
The claim is specific: sauna increases growth hormone sixteen-fold. It shows up in podcast clips, short-form videos, and gym conversations — everywhere the word "protocol" gets used as a verb. The number has a source: a 1986 Finnish study by Leppäluoto and colleagues that measured hormone responses to repeated sauna exposure. [1]
The sixteen-fold spike was documented — peer-reviewed, published, and real.
But the protocol behind it doesn't look like any gym sauna session. Ten volunteers sat in an 80°C sauna for one hour, twice daily, for seven consecutive days — fourteen hours of heat exposure in a single week.
Three details from that original study rarely make it into the viral version. The growth hormone response started declining after the third day — the body adapted within the study's own timeframe. And the researchers themselves raised the possibility that the spike came from water loss rather than a real hormonal change. [1]
The growth hormone spike is real. What it does for your muscles is not.
- Zero of forty studies in a systematic review measured whether sauna builds muscle or changes body composition.
- The viral 16x growth hormone claim traces to a 1986 study using two hours of sauna per day for a full week, nothing like a typical gym session.
- The most-cited sauna health statistics all come from one observational cohort of Finnish men, measured once in the 1980s.
- Twice-weekly sauna use temporarily cut sperm count by 58 percent, but every measure returned to normal within six months of stopping.
- The strongest clinical evidence in the review was for heart failure patients, not for gym-goers chasing muscle.
What Forty Studies Didn't Measure
In 2018, two Australian researchers — Joy Hussain and Marc Cohen — published a systematic review that did what no single study could. They pulled together the entire evidence base for dry sauna bathing. Forty studies. 3,855 people. Twelve countries. Seventeen years of research.
The health outcomes they found covered nearly everything a researcher could measure. Heart failure patients improved after just two weeks of infrared sauna sessions. A twenty-year Finnish cohort linked frequent sauna use to reduced dementia and cardiac death risk. They found effects on chronic pain, headaches, depression, lung function, cholesterol, and sperm quality.
Muscle growth wasn't on the list. Body composition wasn't on the list. Hypertrophy — the process of muscles getting larger — wasn't measured in any of the forty studies.
Not once across forty studies. Not in any of twelve countries.
This is not a negative finding — not a study that tested the claim and came up empty. This is a complete absence. Nobody with research funding thought the connection was plausible enough to test.
Sauna does change something measurable in athletes. It's just not muscle. Seven elite athletes who sat in a sauna for 30 minutes after training every day expanded their blood plasma by nearly 18 percent in four days. That's cardiovascular adaptation, not growth hormone magic.
Where the Logic Chain Breaks
The belief works like a chain. Sauna spikes growth hormone, growth hormone builds muscle, therefore sauna builds muscle.
Two links are already cracked. The spike requires extreme conditions and fades within days. Nobody in forty studies tested the endpoint. But the final link — the part where growth hormone actually builds muscle — is where the whole thing comes apart.
A 2009 study from McMaster University put that final link to a direct test. Eight young men did the same arm exercises under two setups — one that kept hormone levels flat and one that spiked growth hormone roughly eightfold through extra leg work. Muscle biopsies four hours later measured protein synthesis at the source. [2]
The elevated hormones made zero difference. Muscle protein synthesis was identical between conditions. The P-value — a measure of whether the gap was real or noise — came back at 0.72, meaning there was a 72 percent chance the tiny difference they observed was nothing. [2]
The researchers' conclusion was direct: hormone spikes after exercise do not predict muscle growth. [2]
Even if sauna spiked growth hormone at a dose that didn't fade and wasn't driven by water loss — the hormone doesn't do the job the viral version says it does.
Post-exercise hormone spikes cannot be used as markers for muscle growth potential.
The Three Numbers That Built an Industry
There is a second layer to the sauna story — one that has nothing to do with muscle. Three widely shared statistics give sauna its reputation as a longevity tool.
A prospective study followed 2,315 middle-aged Finnish men for over twenty years. Men who reported using sauna four to seven times per week showed a 66 percent lower risk of dementia, 63 percent lower risk of sudden cardiac death, and 40 percent lower all-cause mortality compared to once-a-week users. These are the three numbers behind every sauna brand marketing claim you've ever encountered.
Here is what those marketing campaigns leave out. This was tracking research — tracking correlation, not proving causation. Sauna frequency was self-reported once, by questionnaire, in the 1980s. The groups that used sauna more often also differed in age, alcohol consumption, smoking habits, and physical activity.
McGill University's Office for Science and Society examined the same research group independently. Scientist Jonathan Jarry documented the pattern: the same researchers published twelve papers in a single year from the same Finnish cohort — a practice he called "salami slicing." [3]
The same approach that produced the dementia and cardiac death numbers also produced another claim. Sauna bathing, they reported, was linked to lower psychotic disorder risk.
Of the nineteen heart-related studies in the full review, sixteen came from one research group. Only three of thirteen controlled trials — the kind where people are actually placed into groups — cleared minimum quality bars.
Sperm count dropped 58 percent in three months. Six months after stopping, completely back to normal.
The Side Effect Nobody Mentions
One finding in the review lands differently than the rest. Researchers tracked ten healthy men who used a Finnish sauna for fifteen minutes, twice weekly, over three months.
Sperm count dropped from 223 million to 93 million — a 58 percent reduction in three months. Motile sperm — the ones capable of reaching an egg — fell from 58 percent to 36 percent.
Then they stopped going to the sauna.
Six months later, every measure had returned to normal. Sperm count, motility, all of it — back to baseline. No lasting changes in sex hormones were detected at any point during or after the study.
Ten men. One study. But the mechanism — how testes respond to lasting heat — is consistent with decades of fertility research. The effect is real, and fully reversible, if you know it exists.
What You're Actually Getting From That Bench
The researchers who read all forty studies wrote a conclusion that belongs in every sauna conversation. There is not enough evidence to say which type of sauna works for any given health goal.
That isn't defeat. It's honesty from the people who actually read the evidence base.
What the review did find were effects on the heart. The largest randomized trial — 149 heart failure patients across multiple centers — showed improved walking distance, reduced heart size on imaging, and better functional classification. All of that after just two weeks.
The mechanism wasn't growth hormone. It was hormesis — the body getting stronger through repeated, manageable stress. Heat shock proteins fire up, and the heart adapts toward a new baseline.
Sauna's actual value for someone in a training deficit isn't muscle protection through a hormone spike. It's heart conditioning. It's a nervous system reset between sessions that drain you.
Temperature tools keep driving research beyond sauna alone. A 2021 study found that young men who combined cold-water dipping with regular sauna use burned more calories during cooling than matched controls. But the sample was seven people, the study could not separate cold from heat effects, and the real-world gap was small. [4]
The bench is still worth the time. The reason just changed.
There's another temperature tool in most gym locker rooms. The ice bath has its own research — and unlike sauna, those studies actually measured muscle growth. That story reads differently than this one.
The bench stays in your routine. The story you tell yourself about why you're sitting on it is what changes.
If you or someone in your circle is trying to conceive, the three-month window matters. The sperm impact is temporary and fully reversible, but knowing the timeline turns a surprise into a scheduling decision.
The next time someone in your gym mentions the 16x growth hormone claim, you now have three sentences: the number comes from a 1986 study where ten people sat in a sauna for two hours a day. The spike faded after three days. And a separate study proved that kind of hormone spike doesn't build muscle anyway.
What other research found
What this means for you
The review found that athletes who used sauna after training expanded their blood plasma volume by nearly 18 percent within four days. Their bodies also adapted to heat faster over time, with post-sauna temperature spikes dropping from 2.6°C to 1.9°C after five months.
That's a cardiovascular efficiency gain, not a muscle gain. Sauna after training may help your body regulate temperature and move blood more effectively during hard sessions.
The growth hormone spike the internet talks about was never connected to actual muscle growth in any of the forty studies this review examined.
The biological mechanism behind the sperm drop is heat shock proteins surging during sustained testicular heat exposure. Every marker of that surge was statistically significant.
The practical timeline: three months of twice-weekly sessions produced the impact. Six months after stopping, everything returned to baseline. No lasting changes in sex hormones were detected at any point.
If conception timing matters, that six-month reversal window is the number worth remembering.
The strongest clinical evidence in this entire review belongs to heart failure patients. The largest randomized trial enrolled 149 people across multiple hospitals and found improved walking distance, reduced heart size on imaging, and better functional classification after just two weeks.
The critical detail: this was supervised infrared sauna therapy in hospital settings, not a gym sauna. The protocol, the temperature, and the medical oversight were all specific to the clinical setting.
The review found zero severe adverse events across all forty studies, but the populations with the highest adverse effect rates were people with inflammatory conditions and chronic fatigue.
One of the three highest-quality trials in the entire review tested sauna for chronic tension headaches. Thirty-seven people completed a six-week program, and headache intensity dropped by 44 percent.
That study scored a 4 out of 5 on the quality scale used to evaluate clinical trials. Most studies in this review scored below 3.
The pain story has an honest caveat: a separate study found that sauna reduced pain and stiffness in arthritis patients during treatment, but the benefits disappeared after the sessions stopped. Ongoing sessions may be the price of admission.
Before you change anything
The biggest sauna numbers come from middle-aged Finnish men who were between 42 and 60 when the study started. The cohort was recruited from one region of eastern Finland. Women were not included in the two largest studies.
Most of the clinical trial evidence comes from hospitalized Japanese patients receiving supervised infrared sauna therapy. The athlete data covers just 7 to 16 people. The sperm study followed 10 men.
If you're a woman, under 40, or using an unsupervised gym sauna, the strongest evidence in this review wasn't collected from someone like you.
The type of sauna tested in most clinical trials isn't the one in your gym. The strongest randomized evidence used Waon therapy, a specific Japanese infrared protocol at 60°C for 15 minutes. A typical gym sauna runs at 80 to 90°C. Whether those results transfer across sauna types is an open question the review itself couldn't answer.
The studies were too different from each other to combine mathematically. Different populations, different sauna types, different outcome measures, different durations. The review is a narrative summary, not a pooled statistical analysis. That means no single effect size captures the overall picture.
More than half the studies had 30 or fewer participants. Small studies can find real effects, but they can also find effects that disappear in larger groups.
The case against sauna building muscle is rock-solid. Zero of forty studies measured body composition. That absence is a fact about the evidence base, not a debatable conclusion.
The cardiovascular benefit evidence is real but comes with heavy caveats. The heart failure trial is the strongest piece: randomized, multi-centre, 149 patients, measurable improvements in two weeks. The dementia and cardiac death numbers are impressive but observational, from one cohort, measured once in the 1980s.
The sperm finding is biologically plausible but based on ten men. The mechanism is consistent with known testicular heat sensitivity. The full reversal is reassuring. But the sample is too small to generalize with confidence.
Sauna sits on one side of the gym locker room. The ice bath sits on the other. One tool's muscle claim just collapsed. The other tool's muscle claim has actually been measured.
Researchers pooled every study that tracked cold water immersion and hypertrophy over time. The findings land differently than the sauna story, because this time, somebody actually tested the thing the internet assumes is true.
What This Study Found
All findings from this paper, in plain language.
- Heart failure patients who used infrared sauna for two weeks walked farther, had smaller hearts on imaging, and improved their functional status.
- Finnish men who used sauna four to seven times a week had roughly two-thirds lower dementia risk over twenty years compared to once-a-week users.
- The same group of Finnish men showed 63 percent lower sudden cardiac death risk and 40 percent lower overall mortality with frequent sauna use.
- Heart failure patients who used sauna for two weeks saw their irregular heartbeats drop by more than 70 percent.
- People with blocked leg arteries experienced less pain, walked farther, and showed signs of new blood vessel growth after ten weeks of sauna.
- Sauna reduced pain and stiffness in arthritis patients during treatment, but the benefits disappeared after the sessions stopped.
- Six weeks of regular sauna bathing cut chronic headache intensity nearly in half in one of the review's highest-quality trials.
- Twice-weekly sauna use dropped sperm count by 58 percent in three months, but everything returned to normal within six months of stopping.
- Across all forty studies, no severe side effects were reported. The most common complaints were mild heat discomfort and temporary low blood pressure.
- The review proposed that sauna works through hormesis, the body's stress-adaptation response, triggering heat shock proteins and cardiovascular adjustments rather than muscle-building hormones.
- Sixteen of nineteen cardiovascular studies came from one research group using one specific sauna protocol, raising questions about independent replication.
- Only three of thirteen randomized trials scored above minimum quality thresholds, and more than half of all studies had thirty or fewer participants.
- The review found no clear evidence that one type of sauna is better than another for any health outcome.
- Athletes who used sauna after training expanded their blood plasma volume by nearly 18 percent within four days, a cardiovascular adaptation unrelated to muscle growth.
- A small trial found sauna improved appetite, relaxation, and physical complaints in people with mild depression compared to bed rest over four weeks.