What Actually Happens to Your Body During a Sauna Session

Wellness | 8 min read

Most people who step into a sauna for the first time know roughly what to expect: it's going to be hot, they're going to sweat, and they'll probably feel pretty good afterwards. What they don't expect is quite how much is happening beneath the surface and how profound those changes actually are.

A sauna session is not passive. It is one of the most significant physiological events you can put your body through without exercising. Understanding what's happening, minute by minute, system by system, transforms the experience from a pleasant indulgence into something you approach with genuine respect and intention.

Here's what your body is actually doing in the heat.

The First Few Minutes: Your Body Sounds the Alarm

You step into the sauna. The air temperature is somewhere between 70 and 100 degrees Celsius. Your skin registers the heat almost immediately, and your body's thermoregulatory system, the elaborate network responsible for keeping your core temperature stable, kicks into action.

Within the first two to three minutes, your skin temperature rises rapidly. Thermoreceptors in the skin send urgent signals to the hypothalamus, the region of the brain that acts as your body's thermostat. The hypothalamus responds by triggering a cascade of physiological changes designed to protect your core temperature.

Blood vessels near the surface of the skin begin to dilate; a process called vasodilation. This redirects blood flow towards the skin, where heat can be released into the environment. At the same time, your sweat glands activate. You have around two to four million of them, and in a sauna, they can produce anywhere from half a litre to a full litre of sweat per hour.

Your heart rate begins to climb. What starts at a resting rate of perhaps 60 to 70 beats per minute can reach 100 to 150 beats per minute within a typical session, comparable to a brisk walk or light jog. Cardiac output increases significantly as the heart works to pump blood to the skin's surface.

Your body is working. Hard.

Minutes Five to Ten: The Cardiovascular System Opens Up

As the session continues, vasodilation deepens. Blood vessels that were narrow begin to open substantially. The volume of blood flowing through your cardiovascular system effectively redistributes, up to 50 to 70% of cardiac output is directed towards the skin during peak heat exposure, compared to perhaps 5 to 10% at rest.

This is one of the reasons regular sauna use has such well-documented benefits for cardiovascular health. The repeated cycle of vasodilation and the subsequent constriction when you leave the heat acts as a form of passive cardiovascular training. The vessels become more elastic and responsive over time. Blood pressure regulation improves. The endothelium, the thin layer of cells lining your blood vessels, becomes more efficient at producing nitric oxide, which keeps vessels supple and helps regulate blood flow.

The landmark Finnish study that followed over 2,300 men for more than twenty years found that those who used a sauna four to seven times per week had a 50% lower risk of fatal cardiovascular disease compared to those who used it once a week. The mechanism is, at least in part, exactly this: the heart and blood vessels being repeatedly trained by heat.

Meanwhile, your plasma volume, the liquid component of your blood, begins to shift. Fluid moves from the blood into the tissues and then out through sweat. This is why hydration before and after a sauna matters, and why you feel thirsty afterwards.

Minutes Ten to Twenty: Heat Stress and the Hormonal Response

By now your core body temperature has begun to rise, typically by one to two degrees Celsius over the course of a full session, sometimes more. This is a significant shift. Your body treats it as a genuine physiological stressor, and responds accordingly.

The adrenal glands release norepinephrine and, to a lesser extent, adrenaline. These hormones sharpen alertness and prepare the body to respond. Growth hormone begins to rise, sauna use has been shown to increase growth hormone levels by two to five times baseline, with some studies reporting increases of up to sixteen times in repeated same-day sessions. Growth hormone is central to tissue repair, muscle maintenance, fat metabolism, and cellular regeneration.

Cortisol, the primary stress hormone, rises acutely during the session. This might sound counterproductive for something marketed as a stress-relief tool, but the distinction between acute and chronic cortisol elevation is crucial. The acute spike that occurs during a sauna session is a healthy, adaptive response. It's the chronically elevated cortisol of prolonged psychological stress, sustained over days, weeks, and years, that causes the cellular damage associated with ageing and disease. The sauna's cortisol spike is short, purposeful, and followed by a significant drop below baseline as you cool down afterwards.

This drop is part of what makes you feel so settled after a session.

The Sweat Itself: More Than Just Water

Sweat is often thought of simply as water leaving the body, but it's more complex than that. Sauna sweat is composed primarily of water, but also contains sodium, chloride, potassium, trace minerals, and, in smaller quantities, some metabolic waste products including urea and lactic acid.

The idea that saunas "detox" the body is somewhat overstated; the liver and kidneys handle the vast majority of metabolic waste removal. But sweating does play a meaningful role in thermoregulation and mineral balance, and there is some evidence that certain environmental toxins including heavy metals are excreted in measurable quantities through sweat during heat exposure.

What matters more is what the sweating represents: the body working at full capacity, every system engaged, every mechanism deployed. There is a reason that cultures across the world, Finnish, Russian, Turkish, Korean, Japanese, indigenous North American, have independently arrived at ritual heat exposure as a cornerstone of physical and spiritual wellbeing. The body has always known something about heat.

What Happens in the Brain

The neurological effects of sauna are among the most interesting and least discussed aspects of the experience.

As core temperature rises, the brain increases production of several key neurochemicals. Beta-endorphins, the same molecules responsible for the runner's high, are released, producing a sense of warmth, ease, and mild euphoria. This is why people often describe feeling unexpectedly good towards the end of a session, even through the discomfort of the heat.

Dynorphin, a less well-known endogenous opioid, is also released during heat stress. Dynorphin is thought to be responsible for the slightly uncomfortable, dysphoric quality of intense heat, but its release subsequently upregulates the brain's sensitivity to endorphins, meaning the good feeling that follows is amplified.

Brain-derived neurotrophic factor (BDNF), sometimes described as fertiliser for the brain, increases with heat stress. BDNF supports the growth and maintenance of neurons, plays a central role in learning and memory, and has been linked to protection against depression and neurodegenerative disease.

There is also a profound effect on the default mode network, the brain's resting state circuitry, which is associated with self-referential thinking, rumination, and the kind of mental chatter that characterises anxiety. Heat, like cold and like meditation, appears to quieten this network. The mind, occupied with the immediate physical experience of the body, stops rehearsing its worries. This is not a trivial effect. For many people, it is the most valuable thing the sauna does.

Leaving the Heat: The Recovery Response

When you step out of the sauna, a new phase begins and it is just as important as the session itself.

Core temperature, having risen during the session, now begins to drop. Blood vessels that were dilated begin to constrict. Heart rate gradually slows. The sympathetic nervous system, which has been active throughout the session, begins to yield to the parasympathetic, the rest-and-digest branch that governs recovery, digestion, and deep rest.

Cortisol, which spiked during the session, falls. Endorphins and growth hormone remain elevated. The body enters a genuine recovery state, not the shallow, distracted rest of an evening on the sofa, but a deep, biologically driven return to equilibrium.

This is the window in which so much of the benefit is consolidated. And it is why what you do after the sauna matters. If you follow the heat with a cold plunge, you accelerate and deepen this recovery response, the cold shock drives a further cascade of neurochemical and physiological changes that compound the sauna's effects significantly. If you simply rest quietly, allow the body to cool gradually, sit by a fire and let the mind go still, that recovery is real and deep.

Why Wood-Fired Changes the Experience

There is a difference, and it is not merely aesthetic, between a wood-fired sauna and an electrically heated one.

In a traditional wood-fired sauna like the one at Siidde, the heat builds slowly and unevenly, radiating from the stones and the walls in a way that feels immersive rather than imposed. The air has a particular quality, slightly smoky, rich with the smell of heated wood, alive in a way that processed air is not. The temperature fluctuates gently as logs are added or the door is opened. The sound of the fire is present throughout.

These sensory qualities matter neurologically. The smell of burning wood, the sound of a fire, warmth that emanates from a natural source, these are signals of safety that the human nervous system recognises at a deeply ancient level. They reduce arousal, promote oxytocin release, and create conditions in which the nervous system can genuinely let go. The wood-fired sauna is not just hotter or more authentic. It is a fundamentally different sensory environment, one the body responds to differently.

The Hour After

Most people who use a sauna regularly will tell you that the hour or so afterwards is one of the most distinctive states they experience. A particular quality of calm, not sleepy exactly, but deeply settled. Clear-headed but unhurried. Physically comfortable in a way that feels earned.

This is the body post-effort, post-stress, post-recovery, every system having been worked and returned to baseline, cortisol depleted, endorphins present, the nervous system resting in genuine parasympathetic dominance.

It is, in many ways, the point.

At Siidde, that hour happens beside a lake and a campfire, in a quiet rural landscape that asks nothing of you. The session has done its work. All that remains is to sit with it.