Depression runs hot. In the 1980s, psychiatrists began noticing that patients with depressive symptoms had higher body temperatures compared to people without, and that their body temperatures didn’t ebb and flow as much throughout the day. The more severe a patient’s depression, the higher their temperature tended to be.
Researchers have since noticed that when someone’s depression improves, their body temperature regularizes, “no matter how their depression got better—electroconvulsive therapy, psychotherapy, antidepressants, whatever,” says Ashley Mason, a clinical psychologist at the UC San Francisco Osher Center for Integrative Health. This got Mason thinking: If the two are linked, what happens to a depressed person’s symptoms if you provoke a change in their temperature?
There’s a sizable group of people who already play around with their core temperatures regularly who can help answer this: sauna users, who whip their temperatures up, and in doing so, also push their bodily cooling systems into overdrive. Anecdotally, saunas are associated with elevated levels of well-being, but there’s only been limited research into their impact on mental illness.
In a small 2005 study, sauna sessions appeared to help mildly depressed people recover lost appetite, feel more relaxed, and feel a small improvement in their symptoms. And in a 2016 piece of research, 30 people with major depressive disorder completed a six-week-long clinical trial of infrared “hyperthermia”—that is, raising their temperatures in an infrared chamber—and, again, their depression appeared to wane. These results caught Mason’s eye. “I was bewitched by the finding,” she says. “When you see an effect like that, you don’t really want to let it go.”
That work inspired Mason to investigate the relationship between body temperature and depression more deeply. First, she analyzed seven months of daily temperature readings and monthly depression symptoms from more than 20,000 people to demonstrate the strength of the connection—sure enough, higher body temperatures correlated with depressive symptoms.
Following this, her lab ran a clinical trial to begin testing the antidepressant power of being exposed to extreme heat. Preliminary results of the small study, published Tuesday in the International Journal of Hyperthermia, suggest that a form of heat treatment that Mason’s team calls “whole body hyperthermia” may reduce depression symptoms—potentially significantly.
In the study, 12 adults with depression received eight weekly cognitive behavioral therapy (CBT) sessions, accompanied by at least four heat sessions over this period. Heat sessions lasted up to 140 minutes, or until participants’ core temperatures reached 38.5 degrees Celsius—1.5 degrees above the average human body temperature. Each person had been diagnosed with major depressive disorder based on clinical interviews and a questionnaire known as the Beck Depression Inventory-II, or BDI-II, where higher scores indicate higher severity of depression.
Mason’s team hoped to observe mood changes in their post-treatment interviews, as well as lower BDI-II scores; a three-or-more point drop on the BDI-II is considered clinically meaningful, and CBT trials sometimes reduce BDI-II scores by five to nine points. What they saw was bigger: The participants’ scores fell by an average of 15.8 points. Scores on separate questionnaires about “negative automatic thinking”—involuntarily biased thoughts that are prevalent in depression—also decreased about 20 percent, hinting at potential improvement. And 11 of the 12 no longer met the threshold for major depressive disorder at the end of the eight weeks.
“The premise of this is great,” says Earric Lee, a physiologist with the Montreal Heart Institute, who was not involved in the study but has led sauna studies since completing his doctoral research in Finland. “Trying to move away from pharmacology is a good thing.” More than 250 million people worldwide have major depressive disorder, and tens of millions of people don’t respond to any available treatment.
Such a small study doesn’t prove that sauna therapy can treat depression. “Single-arm studies have meaningful weaknesses,” Mason admits. The cohort was too small to test multiple scenarios, such as varying degrees of heating, CBT without heat, or an attempt at a placebo. (Tricking people into thinking they’ve had heat treatment when they haven’t is difficult, but not impossible—the 2016 study into hyperthermia had a placebo arm that subjected people to mild heat, and convinced 72 percent of participants that they were receiving the actual treatment.)
But these results harden Mason’s hunch that heat sessions may ease debilitating symptoms of depression, and that this is an avenue that needs to be better explored. Eight weeks of CBT alone shouldn’t achieve such high remission rates.
Adam Chekroud, an adjunct assistant professor of psychiatry at Yale University, appreciates the potential benefit of the hyperthermia routine, but remains skeptical about why Mason’s study produced the results reported. For one, some of the participants completed weekly sauna sessions in Mason’s study while others completed fortnightly sessions; Chekroud believes that the benefit of receiving a “higher dose” of heat would manifest itself if the intervention were as strong as effective antidepressants. “The placebo effect is powerful in mental health,” he says.
Still, Chekroud sees the value in these explorations. “So much of the history of the treatments for depression started out as a bit experimental,” he says. Exercise, meditation, and yoga are all potentially effective mind-body options. “Exercise is a phenomenally effective treatment,” Chekroud says, noting his own 2018 study in The Lancet analyzing data from 1.2 million people.
Comparing heat therapy to exercise is not totally unfounded. Beyond just sweating and increasing heart rate, exercise, like using a sauna, also requires getting out of bed, making a plan, and maybe interacting with people—actions that themselves may have an impact on mood. “You’re gonna go back home and feel proud that you’ve made a change in your life,” Chekroud says. “Psychologically, these are big similarities.”
But it’s plausible that the heat itself may contribute too, notes Lee, the sauna physiologist. We know that thermoregulation correlates with mood and moves in tandem with circadian rhythms: Your body typically cools down at night and warms in the early morning, but this temperature regulation turns wonky in severe depression. This might then impair sleep, which is also linked to mood.
Doses of extreme heat might have some sort of hacking or resetting effect on thermoregulation, which then changes mood. Sauna can kickstart the body’s “cool down” programming, Lee says. “You fake it into thinking that it needs to lower its temperature much more,” he says. “So it will get there quickly.”
Whatever the mechanism, heat therapy may be more accessible to people than pharmaceuticals, talk therapy, or vigorous workouts. “We know exercise works for depression,” Mason says. “It’s just much easier to get someone to lay down in a sauna for a while than to go on an hourlong run—especially if they have depression.” Ditto for people with disabilities.
Mason’s trial is still several years—and many grant dollars—away from really nailing down the efficacy of antidepressant heat, though the new results have bolstered her optimism. “A mind and body treatment with that kind of outcome is surely worthy of further study,“ she says. “I hope that grant reviewers and funding agencies will agree.” Her long-term goal is to amass enough convincing evidence for insurance companies to cover practices like sauna, “so that when a person with depression is considering a menu of treatment options, this is on the menu.”