What Happens Inside Your Body During 45 Minutes in an Infrared Sauna

Most people step into an infrared sauna and notice the warmth. They start sweating, feel their heart rate climb, and assume everything important is happening at once. But a 2025 study out of Cardiff Metropolitan University tells a more precise story, one where the body's response unfolds in a specific sequence, on a specific timeline, and understanding that sequence changes how you approach every session.

The study, published in Experimental Physiology, tracked 12 healthy adults through 45 minutes in a 65°C far-infrared sauna. Researchers measured skin temperature, core temperature (rectal), heart rate, blood pressure, sweat rate, plasma volume, and haematocrit continuously or at fixed intervals throughout the session and for 30 minutes afterward.

The result is a detailed physiological map of a single infrared sauna session. And the order in which things happen matters more than most people realize.

First, your skin

Within the first five minutes, mean skin temperature climbed from 35.5°C to 42.9°C, a jump of more than 7 degrees. Then it held. For the remaining 40 minutes, skin temperature barely moved, finishing at 43.0°C.

That rapid rise and plateau is characteristic of how infrared heat works. Unlike a traditional sauna, which heats the air and waits for convection to reach your body, infrared radiation is absorbed directly by the skin. Your tissue reaches its thermal ceiling fast. The body starts mounting a response before the session feels difficult.

Then, your heart

Between roughly minutes 5 and 20, the cardiovascular system does the heavy work. The body detects the rising skin temperature and responds the way it responds to any heat load: it widens the blood vessels near the skin surface and pushes blood outward to radiate heat away.

Heart rate climbs steadily. Sweating ramps up. Blood flow redirects from the core to the periphery. The circulatory system is working progressively harder to dump heat through the skin.

But here is the part most people get wrong about this window: core temperature has barely moved. At the 20-minute mark, core temperature had risen only about 0.2°C above baseline. The cardiovascular response is running well ahead of the thermal one. Your heart is already working hard while your deep tissue temperature is still close to where it started.

This matters because many people step out of an infrared sauna at the 15- or 20-minute mark thinking the session is finished. Physiologically, it is just getting started.

Then, finally, your core

Around the 20-minute mark, the threshold shifted. Core temperature began climbing meaningfully and continued rising for the rest of the session. By the end of 45 minutes, rectal temperature had increased by 1.4°C, from a baseline of 37.3°C to 38.7°C. Tympanic temperature (measured at the ear) rose even further, climbing 2.1°C to 39.1°C.

That 1.4°C core temperature rise is the number that connects infrared sauna to the broader sauna research. It is in the same range documented for traditional sauna sessions, and core temperature elevation is the stimulus researchers point to for the heat-stress adaptations, such as heat-shock protein activity and vascular conditioning, proposed to underlie long-term cardiovascular benefits.

The session was not comfortable. By the end of 45 minutes, participants rated their thermal sensation as "extremely hot" and their discomfort as "extremely uncomfortable" on standardized scales. Two of the twelve participants could not complete the full session, exiting at 40 and 43 minutes. This is real physiological stress, not passive relaxation.

The full picture at 45 minutes

By the end of the session, sitting completely still:

• Heart rate had risen from 74 to 153 beats per minute, more than doubling
• Sweat rate reached 1.4 litres per hour
• Participants lost an average of 1.0 kg in body mass, nearly all of it water
• Plasma volume contracted by 11.6%
• Haematocrit increased from 42.0% to 44.7%

The heart rate finding deserves its own emphasis. A resting rate of 74 climbing to 153 while sitting motionless represents a cardiovascular workload that a 2018 review in Mayo Clinic Proceedings characterized as corresponding to moderate- to high-intensity physical activity. That review, authored by the group behind the 21-year Finnish sauna longevity research, was describing traditional saunas. The Cardiff data now shows that far-infrared sauna, operating at a substantially lower ambient temperature, produces the same magnitude of cardiovascular response.

After you step out

Blood pressure dipped after the session. Diastolic pressure dropped from 77 to 64 mmHg, and mean arterial pressure fell from 90 to 81 mmHg. Systolic pressure remained essentially unchanged.

Within 30 minutes of recovery in a normal-temperature room, blood pressure had returned to baseline. Haematocrit and haemoglobin concentration also normalized, indicating that plasma volume recovered as the body reabsorbed fluid.

That temporary dip followed by a return to baseline is consistent with the acute post-exercise hypotension pattern seen after cardiovascular exercise. It is one of the mechanisms proposed in the longitudinal research for how repeated sauna exposure may reduce hypertension risk over time.

The practical takeaway for the immediate post-session window: stand up slowly and rehydrate. The cardiovascular system has been working hard, plasma volume is contracted by more than 10%, and the body needs fluid before it needs anything else.

Heat load drives the response, not the type of sauna

The researchers' core finding is in the title of the paper itself: infrared sauna is an effective mode of heat exposure that elevates core temperature. The ambient temperature is lower than a traditional Finnish sauna (65°C versus 80 to 100°C), but the physiological response, measured by core temperature rise, heart rate elevation, blood pressure changes, and sweat output, is comparable in magnitude.

This matters because the longest-running sauna research, including the Finnish study that linked frequent sauna use to 63% fewer sudden cardiac deaths and roughly 40% lower all-cause mortality, was conducted entirely in traditional saunas operating at 80°C and above. The question of whether infrared sauna produces the same quality of physiological stress has been one of the open questions in the field.

The Cardiff study does not answer the long-term outcomes question directly. That would require following infrared sauna users over decades. But it establishes that the acute physiological response, the one that the long-term cardiovascular benefits are proposed to flow from, is present. The cascade is the same: skin heats, the cardiovascular system ramps up, core temperature rises, and the body mounts the same thermoregulatory, cardiovascular, and haematological response regardless of how the heat is delivered.

Building a protocol from the data

The study ran for 45 minutes at 65°C. That is at the upper end of what most people will want or need, and the discomfort data confirms it.

Temperature: 45 to 65°C for infrared sauna. The study used 65°C, which is the upper range for most far-infrared units. Lower temperatures still produce the skin-heating and cardiovascular response; the core temperature rise simply takes longer to develop.

Duration: Start with 15 to 20 minutes and build from there. The study data shows that core temperature does not begin climbing meaningfully until around the 20-minute mark. Shorter sessions still produce the cardiovascular response but may not achieve the full core temperature elevation. As tolerance builds, extend toward 30 to 45 minutes.

Hydration: Participants lost 1.0 kg of body mass, nearly all water, and plasma volume contracted by 11.6%. The study did not allow fluid intake during the session, which contributed to the contraction. In practice, drink water before and after, and bring water into the session.

Post-session: Stand up slowly. Blood pressure drops acutely, and plasma volume is reduced. Give the body a few minutes to stabilize before moving quickly.

Frequency: The Finnish longitudinal data found the strongest cardiovascular and mortality associations at four to seven sessions per week. The Cardiff study measured a single session. The protocol the broader evidence supports is consistent, frequent exposure over months and years.

When to check with your doctor: If you have a cardiovascular condition, are pregnant, or have concerns about heat tolerance. The study was conducted on healthy, recreationally active adults. The physiological strain is real and should be treated accordingly.

What this means for your setup

The study used a far-infrared sauna operating at 65°C. That is the type of sauna you can put inside a home and run on a standard household outlet. The accessibility of the format is part of what makes the frequency the research supports achievable.

For infrared sessions with integrated red light therapy panels, the Pod Sauna and Corner Pod operate at 18 to 65°C and run on a standard household outlet. The Pod Lite is another way into infrared heat at home.

For the flexibility to run both infrared and traditional heat sessions, the Hybrid Sauna combines far-infrared heaters with a traditional heating system in one cabin, reaching traditional sauna temperatures for higher-heat sessions while also offering infrared-only use at the lower temperature range the Cardiff study tested.

Browse the full sauna collection to find the right fit for your space and your protocol.

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This content is educational information, not medical advice. Consult your physician regarding your individual circumstances, especially if you have a cardiovascular condition, are pregnant, or have concerns about heat exposure. Sauna use is a complementary wellness practice, not a treatment for any disease.

Study citations

1. Jenkins EJ, Killick JA, Grimm SR, Davies SR, Benson JA, Tremblay JC, Stembridge M. (2025). "Infrared Sauna is an Effective Mode of Heat Exposure That Elevates Core Temperature." Experimental Physiology. https://doi.org/10.1113/EP094028
2. Laukkanen JA, Laukkanen T, Kunutsor SK. (2018). "Cardiovascular and Other Health Benefits of Sauna Bathing: A Review of the Evidence." Mayo Clinic Proceedings, 93(8):1111-1121. https://research-information.bris.ac.uk/ws/files/154526230/Clean_version_Sauna_review_MCP_Final.pdf