Hot tubs deliver the deepest heat
The study involved 20 healthy men and women aged around 24. Each participant completed three separate heat sessions:
Hot water immersion: 45 minutes at 40,5°C
Traditional sauna: 3 rounds of 10 minutes at 80°C
Far-infrared sauna: 45 minutes at 45–65°C
Researchers tracked changes in body temperature, heart function, and immune activity before, during, and after each session.
The results were clear. The hot-tub group showed the biggest rise in core body temperature, about 1,1 °C on average, compared with only 0,4 °C in the traditional sauna and virtually no change in the far-infrared sauna.
Lead author Jessica Atencio explained why: “Hot water immersion gives you the most robust change in core temperature because you can’t dissipate heat through sweating. When you’re submerged, the body can’t cool itself as efficiently, and that’s exactly what drives the beneficial stress response.”
Circulation and immune effects that last
That internal heat load also translated into a more powerful cardiovascular response. Participants’ cardiac output, the amount of blood pumped per minute, jumped by nearly 4 litres in the hot tub, compared with about 2 litres in the sauna.
According to Atencio, that rise in blood flow itself is beneficial: “Increasing body temperature causes an increase in blood flow, and just the force of blood moving across your vessels is good for vascular health.”
Interestingly, only the hot-water sessions triggered measurable immune changes. Levels of the inflammatory messenger interleukin-6 rose modestly after immersion, a normal sign that the immune system had been stimulated. And 24 hours later, participants also showed higher counts of natural killer cells and CD8+ T cells –two key immune defenders.
Why it matters for health and recovery
Senior researcher Professor Christopher Minson, who has studied heat therapies for over two decades, sees clear promise: “There’s no doubt in my mind that if people are willing to do some heat therapy, it’s going to align with improved health, as long as it’s done in moderation.”
Regular exercise still offers broader benefits, but Minson points out that for those who can’t, heat therapy may serve as an alternative way to improve circulation and cardiovascular fitness.
“If you repeat these stresses over time, our lab and many others have shown that they’re consistent with improved health,” he said.
Far-infrared saunas, often marketed as the most “modern” option, turned out to be the least effective in this study. Core temperatures barely changed, and cardiovascular and immune systems hardly reacted.
Which should you choose?
Both traditional saunas and hot tubs are effective tools, but hot tubs are the more potent option. Aim for around 40 °C for 30–45 minutes to mimic the study conditions. Infrared saunas feel pleasant but are the least effective physiologically. They may still aid recovery through relaxation, but the thermal stress is minimal.
Hot tubs may have earned their reputation as a luxury item, but science now backs them as a legitimate wellness tool, improving blood flow, priming your immune system, and offering some of the same benefits as moderate exercise.
