The belief that heat can benefit the mind and relieve psychic suffering has a long history across human societies. But when Eva Hilland scanned the scientific literature for the kind of evidence that would support recommending heat as a legitimate intervention for mental health, she came up with very little.
"There are many knowledge gaps," says Hilland, a cognitive neuroscientist at Oslo Metropolitan University. Although, what existing evidence she did find seemed promising. "So we just saw the potential."
Sauna-going – sometimes paired with cold plunges or winter swimming – is attracting a new generation of followers, from the U.S. to Hilland's sauna-steeped country of Norway.
The trend not only reflects the reach of the broader wellness and longevity movement, but also a thirst for communal experience and social connection, particularly in the wake of the pandemic.
The science is just beginning to catch up with many of the theorized benefits. Though considerably less studied than the cardiovascular impact, mental health has emerged as one of the more exciting and appealing applications.
Sauna survey
Hilland, whose research group focuses on neuroimaging and genetics, is one of the scientists now diving into this research domain of heat and mental health. And she's leveraging the sauna popularity surge in her own city, Oslo, to do it.
With funding from a local municipality, she's recruiting 50 people for a randomized controlled trial to see whether coupling sauna with a simple set of mental health habits improves stress, well-being, sleep and more.
Instead of visiting a windowless research lab, her study participants take two sessions a week at one of the floating sauna villages that have cropped up in Oslo's inner harbor – a waterfront that has transformed into a world-class destination for heat-seekers in just the past decade.
Large surveys of sauna users reflect what many readily share: It's the psychological benefits more than anything else that keeps them coming back, whether that's stress reduction, elevated mood, improved sleep or the ineffable post-sauna glow.
Some of these self-reported improvements may have less to do with sweating and more to do with the digital detox, time in nature, mindfulness and social connection that sauna-going facilitates.
But the weight of the evidence suggests heat itself is also "powerful physiological stimulus for promoting mental health," says Christopher Lowry, an integrative physiologist and neuroscientist at the University of Colorado, Boulder.
What we know heat does
While warm baths were commonplace in psychiatric asylums a century ago, the modern evidence base largely rests on translational work done with animals and cell models, and a small number of human studies, most of which lack strict control groups.
In his lab, Lowry has extensively explored the physiological mechanisms linking temperature and mental health.
When our skin senses heat, those signals travel to a primitive part of the brain that tells our body to cool down. That process overlaps with a pathway involved in mood and pleasure, which is implicated in depression.
Heat also triggers a specific anti-inflammatory response. A similar phenomenon occurs during exercise, a well-established treatment for depression and other mental health conditions. And separately, Lowry says, animal studies find heat activates a group of "serotonin neurons" in the brain that may "provide stress resilience and antidepressant effects."
In humans, the strongest data come from a landmark trial, published ten years ago, and led by a team at the University of Wisconsin-Madison and co-authored by Lowry.
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A total of 30 people with depression were randomly separated into two groups. About half spent around two hours in a whole-body infrared machine designed to raise their body temperature to 38.5 C, or about 101 F. The rest received a sham treatment.
Within a week, a majority of those in the treatment group had a 50% reduction in their symptoms and the effects persisted throughout the six-week follow-up period.
Training the body to cool down
Those findings are what originally inspired Ashley Mason to enter the field.
A clinical psychologist who studies mood disorders and sleep at the University of California, San Francisco, Mason now has funding from the National Institutes of Health and others to study various forms of heat therapy for depression, sleep and loneliness.
The renewed scientific interest in using heat for psychiatric conditions, in part, reflects an intriguing observation – people with depression tend to have difficulty with thermoregulation.
"What we know from a bunch of literature is that depressed people seem to run hot," says Mason, also a professor of psychiatry and behavioral science. "When their depression improves, their thermoregulatory system seems to also change."
In fact, Mason's lab recently tracked this association between self-reported symptoms of depression and body temperature using wearable device data collected from close to 20,000 people.
Even when controlling for factors like weather and medical conditions, symptom severity correlated with higher skin temperature and less daily fluctuation in body temperature, meaning their bodies weren't cooling down as much at night.
It may seem counterintuitive to heat folks up even more, but Mason says doing so seems to function like a "thermoregulatory workout" – training their system to become more efficient at cooling down in the long run.
"So when you are actually done being exposed to that heat, your body temperature drops for a much longer period of time than you were spending in that heat," she says. "Days, possibly weeks, we still don't know."
Several older studies have also shown that successfully treating people with antidepressant medication or other therapies improves their thermoregulation.
The implication, Masons says, is that acting on this "bi-directional" relationship – both via heat and traditional therapeutic approaches – could have a synergistic effect.
To test this hypothesis, she has run two small pilot trials combining eight weeks of cognitive behavioral therapy and four sessions in an infrared hyperthermia machine. In both studies, most of the participants were in remission from depression by the end of the study.
Her team is now carrying out the same protocol with a larger pool of patients at UCSF, Harvard and the Cleveland Clinic.
Mason stresses the results should be viewed as preliminary. The studies were not powered to show the treatments were effective, but they do lay the groundwork for the larger, well-controlled trials that could deliver that kind of evidence.
"Heat does seem to be a promising therapy, but we're very much at the beginning of this research," she notes.
If these future studies pan out, it may have a place alongside other non-pharmacological approaches for mental health, like exercise – at least in some cases.
"For people with depression who have tried a lot of different types of treatment, do I think that there's room for heat therapy to have a seat at the table?" she says. "Absolutely."
Exactly how you get hot may not matter so much as long as people are sweating.
However, the boom of interest in age-old forms of heat therapy like sauna may be a natural entry point for many.
"The science is new, but the practice is not," Mason says.
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