What Actually Happens Once You Close the Sauna Door?

Most mornings, I stop at my local health club on the way to the showroom. Usually I'll spend fifteen or twenty minutes in the steam room before heading to work, but every now and then I use the sauna instead.

It was during one of those sessions that a question occurred to me:

What actually happens once you close the door?

Not over months or years of regular sauna use, I mean during that session. The room is hot, you've sat down on the bench, and your body immediately starts responding. But in what order? What happens first? And how much of that sequence has actually been measured, rather than simply inferred?

These seemed like the sort of questions that somebody would have already answered somewhere - after all, sauna physiology has been studied for decades. But as it turns out, I couldn't find a single study that answered those questions in quite that way - not because the science is lacking, but because that's not how the research is organised. One study might measure heart rate and blood pressure during sauna bathing, another looks at skin temperature, yet another follows changes in blood flow or sweating. Heat shock proteins appear in a different body of research again. Each study answers its own question very well, but none sets out to describe everything that's happening from the moment you walk in until you walk back out.

That turned out to be far more interesting than I'd expected because it meant there wasn't a single timeline to find - there was more of a story to piece together.

Where does the body's response begin?

Like most people, I'd always thought sweating was where the story began. It makes intuitive sense because it's the first thing we're aware of. You sit in the heat for a while, you start sweating, and from that point on it feels as though your body has begun reacting. But by the time you notice the first bead of sweat, your body has already been quietly responding to the heat. Sweating isn't where the body's response begins, it's simply the first part we can see.

The process begins almost as soon as your body detects the increase in temperature. Specialised temperature receptors in the skin register the change in your surroundings and send signals to the hypothalamus, the small region of the brain responsible for regulating body temperature. Its job is simply to stop your core temperature rising too far, despite the fact you've just walked into a room that's considerably hotter than you are.

Blood vessels close to the surface of the skin start to widen, allowing more warm blood to move away from the body's core. Your heart rate also begins to rise as your circulation changes. If someone hadn't explained it to you, you'd probably have no idea any of this was happening, but researchers can measure it and, by the time most of us think our bodies have started reacting to the heat, they've actually been responding all along.

Until I started reading the research, I'd imagined heat moving steadily through the body. Skin first, then muscles, then eventually everything else. That isn't really what's happening.

The research consistently shows that skin temperature rises quickly because it's directly exposed to the hot air. Your core temperature, however, changes much more slowly. In most studies it takes several minutes before researchers detect a meaningful increase, although exactly how long depends on the temperature of the sauna, the humidity, how long you're staying in there and, of course, the person sitting on the bench.

By widening blood vessels, increasing blood flow to the skin and eventually producing sweat, your body is buying itself time. Those responses don't stop the heat altogether, but they do slow the rate at which your core temperature rises. In other words, the body isn't simply heating up - it's also trying its best not to.

That was another point where I realised my mental picture wasn't quite right. I'd imagined the body gradually heating up from the outside in. Instead, the evidence suggests it's spending the first part of a sauna session actively limiting the rise in core temperature. Eventually that temperature does begin to increase, but only after several protective mechanisms have already been working for some time.

That also explains why trying to build a minute-by-minute timeline turned out to be such a frustrating exercise. Two people can sit in identical saunas for the same length of time and still respond differently. A regular sauna user will often respond differently from someone who's new to heat exposure. Hydration, age, fitness and even the time of day can all influence how quickly different responses appear.

By this point I'd stopped asking, "What happens after ten minutes?" and started asking, "What tends to happen before what?" Once I looked at it that way, the rest of the research started making a lot more sense.

Up to this point, I'd been reading studies that were all measuring things researchers could observe directly. Heart rate. Skin temperature. Blood pressure. Even core temperature with the right equipment. Different studies reported slightly different figures depending on the sauna, the participants and the way the experiment was designed, but they were all observing the same physiological changes as they happened. That was where I felt reasonably confident that I understood not just what was happening, but how we knew it was happening.

Heat shock proteins

Heat shock proteins appear in almost every discussion about sauna physiology, and for good reason. They play an important role in helping cells cope with stress by protecting and repairing other proteins that might otherwise become damaged. They're fascinating, but what struck me wasn't the biology itself. It was the way researchers study them, because unlike heart rate or skin temperature, heat shock proteins aren't something you can simply watch changing minute by minute while somebody sits in a sauna.

Instead, much of the evidence comes from comparing biological samples taken before heat exposure with those taken afterwards, sometimes immediately afterwards and sometimes during the recovery period. That tells us a great deal about what sustained heat exposure does inside our cells, but it doesn't tell us exactly when those changes begin.

It's an important distinction because it's tempting to imagine heat shock proteins suddenly appearing at a particular point during a sauna session. The evidence doesn't really support that. Instead, it supports the conclusion that they're part of the body's wider response to heat, sitting alongside the cardiovascular and thermoregulatory changes already underway.

Once again, I realised I'd been asking the wrong question. I'd spent quite a while trying to work out when heat shock proteins entered the picture, but the evidence was nudging me towards a different way of thinking. Multiple systems are responding at the same time, some almost immediately, others becoming measurable only later, and many continuing well after the sauna session has ended. From that point on, I stopped looking for exactly when each response began, what mattered was how they all worked together.

What happens after you leave?

One of the unexpected things I found while reading the research was how much attention it paid to the recovery period. I'd been thinking almost entirely about what happened inside the sauna itself. Once you stepped back into cooler air, I assumed the body simply started returning to normal and that was largely the end of the story. In reality, that's another over-simplification.

Your heart rate doesn't suddenly drop back to where it was before you walked in. Your body continues shedding heat as it gradually returns towards its normal resting state, and researchers have shown that some of the physiological and cellular responses to heat continue into the recovery period.

It also reinforced something I'd been noticing throughout the research. The body's response to heat isn't a series of isolated events with neat beginnings and endings. It's a continuous process of adjustment. Some responses begin almost immediately, others take longer to become measurable, and some continue after the source of the heat has been removed.

So what does actually happen once you close the door?

Not everything happens in a neat timeline, but there is a clear order to the body's response.

It begins with temperature receptors in the skin detecting the sudden change in your surroundings and alerting the hypothalamus. Blood vessels near the surface begin to widen, your heart starts working a little harder, and your body does everything it can to stop your core temperature rising too quickly. Skin temperature climbs rapidly while your core temperature increases much more slowly. Sweating joins in later, helping to remove heat as your body's other cooling mechanisms continue working. Alongside all of this, deeper cellular responses are also developing, although exactly when they begin is much harder to pin down. Even after you leave the sauna, many of those processes continue as your body gradually returns towards its normal resting state.

That's not the tidy minute-by-minute timeline I'd expected to find, but it does answer the question I started with. The body isn't reacting to heat one system at a time. It's coordinating dozens of responses, some almost immediate, some delayed, all working towards the same goal of keeping your internal temperature under control.

I'd started the article thinking of the sauna as the event and recovery as two different things. By the end, I wasn't really thinking about them as separate at all.

The next time I step into a sauna, I doubt I'll spend the whole session thinking about thermoreceptors or heat shock proteins. But I will know that long before I notice the first bead of sweat, my body has already been quietly getting on with the job of keeping my internal temperature under control.

Further Reading

If you're as curious as I was, these are some of the papers and resources I found most useful while researching this article.

• Benefits and Risks of Sauna Bathing - Hannuksela ML, Ellahham S. The American Journal of Medicine, 2001.
• Association Between Sauna Bathing and Fatal Cardiovascular and All-Cause Mortality Events - Laukkanen JA et al. JAMA Internal Medicine, 2015.
• Acute Effects of Sauna Bathing on Cardiovascular Function - Laukkanen T et al. Journal of Human Hypertension, 2018.
• Passive Heat Therapy Improves Endothelial Function, Arterial Stiffness and Blood Pressure in Sedentary Humans - Brunt VE et al. The Journal of Physiology, 2016.
• Sauna as a Valuable Clinical Tool for Cardiovascular, Autoimmune, Toxicant-induced and Other Chronic Health Problems - Crinnion WJ. Alternative Medicine Review, 2011.
• NCBI Bookshelf - useful reference material for human physiology, thermoregulation and cellular stress responses.