Four days each week, after around 20 minutes in an infrared sauna, Pure Sweat Sauna Studio owner Kevin Gillam submerges his body into a 48-degree pool. When he resurfaces, he begins breathing through his nose, eyeing a stopwatch while music blasts from a Bluetooth speaker. “The first 20 or 30 seconds, it’s hard,” he says. He’ll stay in the pool for five minutes before he returns to his sauna, just to repeat the process again and again. 

Gillam is a devout believer in the power of contrast therapy, enough that he opened his own sauna and cold plunge business on North Williams in June 2023. Traditionally the practice involves time in a sauna followed by a cold plunge, though anyone who has ever gone from a hot tub to a pool has engaged in a form of contrast therapy. Practitioners say it helps them relax and creates a sense of well-being that lasts for days after. “It carried over to other parts of my life,” Gillam says. “I sleep way better, I recover from my workouts quicker, my mood increases.”

According to Google Trends, searches for “contrast therapy” have quadrupled since 2020, and searches for “cold plunge” have increased more than tenfold since early 2022. Taking their cue from professional athletes, gyms have started installing their own cold-water baths and cryotherapy chambers. These businesses all readily list the health benefits associated with saunas, hot tubs, and cold-water plunges: lower blood pressure, decreased risk of heart disease and dementia, reduced inflammation, improved insulin sensitivity, and better mental health. These are bold claims—and broad ones. But science is rarely cut-and-dried. 

It’s Physiology 101: When your body experiences heat, it tries to regulate itself. Your blood vessels dilate, carrying blood away from your warm core to the surface of your skin, where it cools down. The impacts of cold, particularly cold-water immersion, are much more profound. The temperature triggers your vagus nerve, which may tell your body to relax. But at the same time, your heart starts to race. You may struggle to control your breathing. Your body might start producing cortisol and adrenaline, hormones that regulate stress. Your blood vessels shrink—this is called vasoconstriction—and in turn, your blood pressure will rise.

These responses are well documented and well studied. Cold-water shock is a common cause of drowning. But if you’re somewhere safe, and you stay in the water long enough, the shock passes. And something…shifts. And that’s where the science gets messy.

A wealth of qualitative and anecdotal research suggests contrast therapy and cold plunges can improve mental health and well-being. Many of these studies have small sample sizes, and studies on self-reported mood can be prone to bias; still, researchers say the sheer number of such studies gives them credence. But the reason people feel better is much murkier. One study, published in 2000, found that levels of noradrenaline and dopamine increased after a cold plunge, while cortisol decreased. But there have only been a small number of studies investigating the association between cold-water immersion and different hormones, and some haven’t found the same reaction.

John Kelly, a senior lecturer of physiology at the University of Chichester in the UK, has studied the impacts of cold-water immersion on mood. In one study, 66 percent of the participants spent 20 minutes up to their necks in the ocean. A week later, they reported better moods than participants who didn’t take the plunge. Kelly believes that a rapid chill “changes the balance between your parasympathetic and your sympathetic nervous system”: Your sympathetic nervous system helps you respond to stress, while your parasympathetic nervous system helps you conserve energy and relax. Both cold water and deliberate breathing can stimulate your vagus nerve, which is a part of that parasympathetic system. “It’s like yoga,” Kelly says. “You’re controlling your breathing.” 

But despite numerous studies with similar findings, Kelly says that “we’re still miles away from a good understanding of it.” 

When it comes to the physical impacts, things get even more complicated. A lot of money has been spent studying the impacts of heat and cold on sports injuries. But research doesn’t always lead to consensus, and temperature is no exception. Even icing an ankle after injury can be controversial. Cold reduces inflammation, and inflammation is implicated in a number of diseases. But inflammation is also an important part of healing.

“In a clinical setting, we get questions all the time like, ‘What’s better, hot or cold?’” says Ryan Petering, a practicing doctor who teaches sports medicine at Oregon Health & Science University. “But we have very poor evidence, in general, for the outcome of these therapies.”

For every study showing postexercise cold plunges can prevent muscle damage, there’s another finding no impact whatsoever. Review articles, which analyze several studies and attempt to draw conclusions, struggle to find high-quality research to include. One 2013 study on contrast therapy found that just 18 papers met their criteria, and all of them “had high risk of bias” due to researcher conflicts of interest and study design.

Some studies have shown that repeated cold plunges can help regulate insulin and trigger important metabolic changes in fatty tissues, but that might just be because shivering impacts your body in ways that are sort of like exercise—your muscles move, after all. Others found that cold-water swimmers could have stronger immune systems, but it was impossible to untangle the potential benefits of cold from exercise here, too.

In the absence of data, Petering encourages his patients to listen to their bodies. “If you have arthritis and warmth isn’t working, consider cryotherapy.” If using the sauna and taking a cold plunge helps you feel better after a workout, keep doing it. And that ankle? There’s a difference between swelling and inflammation, says Petering, and while inflammation can help heal some injuries, swelling can slow healing. If swelling is causing pain, ice it.

To find richer, longer-term studies on contrast therapy, you have to go to Norway, Sweden, and Finland. That makes sense: Sauna originated centuries ago on the Scandinavian peninsula. The traditional Scandinavian or Finnish sauna experience is often coupled with a plunge into an ice-cold lake or the Baltic Sea.

In one Finnish study, researchers tracked more than 2,300 middle-aged men for 20 years. Those who used a sauna four to seven times a week (some with cold plunges, and some without) were two-thirds less likely to die from cardiovascular disease, and were also 40 percent less likely to die from any cause.

The health benefits from sauna might come from the heat and the cold. It’s also possible that the Finnish study’s results are just a correlation. People who use a sauna regularly may be more likely to have the resources to take care of their health in general. But there’s a third hypothesis: Having strong communities and spending time in nature are both associated with reduced mortality. And traditional saunas often involve both.

Some of Kelly’s students conducted unpublished research on the impacts of cold showers and baths on mood. Showers didn’t seem to do much. Cold baths helped a bit. But a dip in the sea had the biggest effect. And in Kelly’s research, much like in traditional sauna practice, the cold plunge is a group activity. But Kelly says it’s hard to determine whether the cold or the social interaction provides the mood boost. His best guess is a combination of the two. “People don’t go into cold water on their own.”