Sauna enthusiast Michael Calcagno, second from right, sits inside a clear-walled wood-fired sauna during the Seattle Sauna Festival in November. Sauna use has taken off as a health practice and scientists are studying its benefits. Mike Kane for NPR hide caption
Saunas have captured the wellness zeitgeist, attracting a new generation of followers eager to tap into the benefits of the age-old practice.
The upswell of public interest has fed a growing industry dedicated to offering the sauna experience, and inspired festivals in cities like New York, Minneapolis and Seattle.
"It is my mental reset – my control, alt, delete," says Ana Hernandez, who launched a mobile sauna business after the pandemic and organized Seattle's first ever sauna festival this past fall.
Turnout for the two-day event ended up being three times what they'd anticipated. Attendees milled around a makeshift sauna village, assembled outside the Nordic Museum in Seattle, sampling different styles of sauna, and occasionally dipping in an ice cold tub or shower.
"The crowd is very mixed," she says, "People come for their physical and mental wellness."
Sauna challenges the body in ways that are similar to exercise, researchers say. Confronted with the sudden increase in heat, your cardiovascular system is put to the test – blood vessels dilate, heart rate increases and blood gets pushed to your skin, where it can be cooled more easily by sweat.
JJ Meston, right, and Chris Shotwell, both from Seattle, exit a sauna after a session during the Seattle Sauna Festival. Heat can boost the cardiovascular system in similar ways to exercise, research finds. Mike Kane hide caption
"There's very good evidence now that repeated use of heat is healthy for humans," says Christopher Minson, a human physiologist at the University of Oregon who focuses on thermoregulation and health.
"We have this incredible ability to adapt to heat that's really helped shape human evolution much more than our ability to adapt to cold," he says.
The most compelling data come from large population studies out of Finland, a country of about 5 million that famously boasts more than 3 million saunas.
A 2015 study, published in JAMA Internal Medicine, that followed more than 2,300 Finnish men for about 20 years has received considerable attention. That showed using a sauna four to seven times per week was associated with a 40% to 60% lower risk of cardiovascular disease and death, compared to those who only went once a week.
Subsequent studies have bolstered those findings on cardiovascular health, showing improvements in blood pressure, cholesterol, arterial stiffness and other markers of cardiometabolic health.
"The evidence is robust, it's consistent," says Dr. Setor Kunutsor, a cardiologist at the University of Manitoba, in Canada, who has been involved in much of the research in Finland.
"We know temperature has an effect on disease, but we were surprised by the magnitude of the effect," he says.
Amanda Morrow, of Seattle, ladles water onto sauna rocks inside a vintage camper trailer-converted sauna during the festival. Mike Kane for NPR hide caption
Traditional Finnish saunas are generally kept anywhere from 180 to more than 200 degrees Fahrenheit. Stepping into a room that hot triggers an immediate stress reaction, activating the sympathetic arm of the nervous system, the fight-or-flight response.
"We see an increase in blood pressure and heart rate" comparable to physical activities like light jogging, says Sascha Ketelhut, an exercise scientist at the University of Bern in Switzerland who has done work in this area.
As with exercise, this "acute stressor" is then followed by improvements in these markers of cardiovascular health and a calming of the nervous system during the recovery period, he says.
While cardiovascular health has the most supporting data, large-scale studies have also linked sauna use to lower rates of respiratory illness and even some neurodegenerative conditions like dementia and Alzheimer's.
There are a number of mechanisms that could explain the decreased risk of chronic disease, among them the effects on systemic inflammation and oxidative stress.
In a 2018 study, Kunutsor and his colleagues showed that Finns who frequently sauna have lower levels of inflammatory markers.
Experiments have revealed that sauna and other forms of heat therapy also cause the secretion of various hormones and boost immune cells, at least in the short-term.
The role of heat shock proteins is of particular interest.
Sauna enthusiasts sit inside a barrel sauna during the Seattle Sauna Festival. According to Christopher Minson, a human physiologist at the University of Oregon, growing evidence shows that regular sauna use can combat inflammation. Mike Kane for NPR hide caption
Minson says these help combat harmful molecules called reactive oxygen species that can build up inside of us and trigger a cascade of inflammation.
The evidence base has grown considerably in the last decade. But Kunutsor says there's still a need for large, well-controlled trials, particularly in populations that are "sauna naive," to tease out the effects. He thinks more evidence may lead medical societies to consider incorporating sauna use into official health guidelines.
For many sauna enthusiasts, the biggest draw is how it makes them feel.
As with the cold plunging craze, saunas increasingly serve as a kind of social lubricant, a place where people can find connection and a brief reprieve from their phones.
However, the link to mental health also has a physiological basis that researchers like Dr. Charles Raison are trying to understand.
"High heat administered for a time-limited period is an antidepressant and a pretty good one," says Raison, a professor of psychiatry and human ecology at the University of Wisconsin-Madison.
This research spans various forms of heat therapy, not just traditional Finnish saunas.
Sophia O'Connor, of Bellingham, Wash., rinses off after a sauna session, while participating in the Seattle Sauna Festival. Mike Kane for NPR hide caption
For example, Raison's group uses a special hyperthermia machine so participants can reach a core body temperature of 101.3 Fahrenheit – hotter than you could typically achieve in a sauna. Their heads, cooled with ice packs, stick out of the machine.
The data gathered so far suggest a linear relationship: The hotter a person gets, up to a certain point, the less depressed they feel in the following days and weeks.
"These studies are small, but the signal is pretty clear," he says.
For example, a randomized-controlled trial of about 30 people, published in 2016, found significant reductions in symptoms of depression after just one session of whole body hyperthermia, compared to a group that received a sham treatment.
"The pathways in the brain and body that mediate thermoregulation overlap spectacularly with the pathways that mediate mood, desire, the state of emotions," Raison says.
An intriguing finding from this research relates to an immune-signaling molecule called Interleukin-6, or IL-6. An acute spike in IL-6 following heat exposure appears to correlate with how much depressive symptoms improve.
More broadly, scientists interested in leveraging heat for mental health have noticed a link to body temperature, specifically that people with depression tend to run hotter, according to a 2024 study of more than 20,000 adults.
