Written by: Lauren Hammond
Published on: November 5, 2026
What the heat is doing to a stressed body
A randomized, sham-controlled trial published in JAMA Psychiatry found that a single session of whole-body heat produced a reduction in depressive symptoms that was still measurable six weeks later (Janssen et al., 2016). That is a real result from a well-designed study, and it is about heat rather than about any particular cabin.
Sleep is where people are most emphatic. In the Global Sauna Survey, an international study of 482 sauna users, 83.5% reported sleep benefits (Hussain, Greaves & Cohen, 2019). That is a large majority of actual users telling researchers the same thing, and it is the single most consistent finding in the sauna literature on sleep.
What has not caught up is the instrument data. The one controlled trial that put sensors on people and measured their sleep after an infrared sauna session found no difference in sleep duration, sleep quality or overnight heart rate variability (Ahokas et al., 2023). It was a small study of 16 athletes measuring a single session, so it is not the last word on the question. It is simply the only objective measurement anyone has published, and it did not detect what the survey data would predict.
So the accurate answer is that an infrared sauna is well supported for how stressed you feel and how rested you feel, that the mood evidence is genuinely strong, and that the gap between 83.5% of users reporting better sleep and one small trial finding no measurable change is a gap worth being honest about. Our read is that the survey data reflects something real. We would rather show you both halves than quote one and hide the other.
Stress is not only a mood. It is a physical setting. When you are under pressure your body sits in what physiologists call the sympathetic state, the one built for responding to threat. Heart rate climbs, breathing shortens, muscles hold tension, and the hormone cortisol runs higher than it should for longer than it should.
The opposite setting is the parasympathetic state, the one your body uses for rest, digestion and repair. Most people spend far too little time there, and the reason a sauna session works is that it gives your body an uncomplicated reason to switch over. You are warm, you are still, you are not doing anything, and there is nowhere to be.
Infrared gets you there at a gentler temperature than a traditional sauna does. An infrared sauna warms the air in the cabin and also delivers energy directly to your body, which a traditional sauna cannot do. Because some of the heat arrives at you rather than only around you, the same physiological response happens without the air needing to reach 180°F.
That difference is measurable. In a Finnish study comparing a 30-minute far-infrared session with a traditional sauna at the same air temperature, heart rate immediately after the infrared session was 71 beats per minute, compared with 92 after the traditional session (Mero et al., 2015). The researchers described far-infrared bathing as “a very light loading for the body” that “provides a comfortable and relaxing experience.” For anyone whose goal is to come down rather than be put through something, that is the relevant finding.
The strongest evidence here is about mood
The JAMA Psychiatry trial is worth understanding properly, because it is unusual in this field and because it is frequently described badly.
Researchers randomized adults with major depressive disorder to either whole-body hyperthermia or a sham condition designed to mimic everything about the real treatment except the intense heat. Both groups were blinded, and the sham was convincing enough that 10 of 14 people who received it believed they had received the real thing. The active group reached a core body temperature of 38.85°C. The sham group reached 37.69°C.
A single session produced significantly lower depression scores than sham at one week, two weeks, four weeks and six weeks afterward (Janssen et al., 2016). The effect sizes were large.
Here is what that study is and is not. It is the best evidence that exists for heat affecting mood, and the sham control is what makes it credible, because it rules out the explanation that people simply felt better because something pleasant was done to them. It is also a small trial, 34 people randomized, using a clinical hyperthermia device in a medical center, in people diagnosed with major depression. It was not an infrared sauna, and the participants were not healthy people looking to decompress after work.
What it supports is the mechanism. Warming the body meaningfully affects the systems that regulate mood. What it does not support is any claim that a sauna session treats depression, and we are not going to make one.
Sleep: what users report, and what instruments measure
The honest picture has two halves that point in different directions.
On one side, people consistently say sauna use helps them sleep. In the Global Sauna Survey, an international questionnaire completed by 482 sauna users, 83.5% reported sleep benefits (Hussain, Greaves & Cohen, 2019). That is real information. Self-reported sleep quality is not a trivial outcome, and it is often exactly what people care about. It is also worth knowing what that number is and is not: a cross-sectional survey of people who already use saunas, covering all sauna types rather than infrared specifically.
On the other side, when researchers measured sleep objectively after an infrared sauna session, they did not find what the surveys would predict. In a randomized crossover trial of 16 athletes, a 20-minute infrared sauna session after training improved next-day muscle soreness and perceived recovery. Overnight heart rate variability, sleep duration and sleep quality showed no difference compared with sitting in a room at normal temperature (Ahokas et al., 2023).
There is a detail in that study worth knowing, because it changes how you should use the sauna. During and immediately after the session, heart rate went up and heart rate variability went down. In the short term a sauna session is a mild stressor, which is the whole point of heat exposure. The calm arrives afterward, not during. Anyone promising you that the session itself drops you into a parasympathetic state is describing the end of the session, not the middle of it.
The mechanism people most often cite for sleep is the cooling-down period. Your core temperature rises during a session and then falls afterward, and a falling core temperature is one of the signals your body uses to initiate sleep. Internal testing on a Sunlighten Solo cabin measured one person’s core temperature every minute and found it rose almost 3°F across a 30-minute session and stayed elevated for roughly another 30 minutes afterward (Sunlighten Core Temperature Study, internal single-subject data). The broader passive body heating literature, which looks at warm baths and showers before bed, supports the same general principle (Haghayegh et al., Sleep Medicine Reviews, 2019).
That is a plausible and well-reasoned mechanism. It is not the same thing as a trial showing an infrared sauna improved measured sleep, and we are keeping those two categories separate.