You’ll often read that infrared heat penetrates 3 to 4 cm into the body. A 2015 sauna study repeated that figure, and a 2025 study tested it with temperature probes in muscle.
This page covers what those studies found. For the health research built on that heat, see the overview of infrared sauna benefits. For settings, see the guide to infrared sauna temperature.
Short answer: it depends on the cabin temperature. At 65°C (149°F), a 2026 study found rectal temperature rose 1.4°C in 45 minutes. A 2025 study measured no change in core temperature over 45 minutes, while thigh muscle warmed by 1.1 to 3.0°C depending on depth. The heat’s effect was negligible beyond 3.8 cm under the skin.
How deep the heat goes
Reed (2025) inserted temperature probes into the thigh muscles of 10 adults, then sat them in a commercially available far-infrared sauna for 45 minutes, in the Journal of Applied Physiology.
| Depth below skin | Muscle temperature rise |
|---|---|
| 1.4 cm | +3.0°C |
| 2.4 cm | +1.9°C |
| 3.4 cm | +1.1°C |
| Core (intestinal) | No change |
The authors wrote that it “has been claimed” infrared waves penetrate 3 to 4 cm into the body. They found the warming lessened with depth and was negligible beyond 3.8 cm. By 2.4 cm, the effect had dropped by 63%, which they called the effective thermal penetration.
When the cabin is hotter
Jenkins (2026) set a far-infrared cabin to 65°C (149°F), closer to traditional sauna heat, and had 12 healthy adults sit for 45 minutes, in Experimental Physiology. The authors noted that earlier work at about 45°C had found limited heat build-up in the body.
At 65°C, rectal temperature rose from 37.3 to 38.7°C. Heart rate went from 74 to 153 bpm, and people sweated about 1.4 liters an hour. Two of the 12 stopped early because of extreme discomfort. The authors concluded:
“The effectiveness of FIR sauna to increase core temperature is governed by the thermal load imposed.”
Jenkins EJ et al., Experimental Physiology, 2026
FIR stands for far-infrared.
They also noted that at this heat, warm air adds to the radiant heat, so the effect can’t be credited to infrared alone.
Is it like exercise?
Hussain (2022) compared 45 minutes of rest, infrared sauna, and indoor cycling in 10 healthy women, in Complementary Therapies in Medicine. Ear temperature rose more in the sauna than at rest. The authors concluded that the effects of infrared sauna come from the body’s response to heat “more so than exercise-mimetic” heart and blood vessel activation.
Limitations
Each study had 10 to 12 people and a single session. The 2025 abstract did not give its cabin temperature, and the three studies used different cabins and temperatures. Core temperature was measured in different ways: intestinal pill, rectal probe, and ear.
Here’s what the spec sheet won’t tell you. A cabin’s “deep penetration” claim and its temperature setting are separate things, and in the 2026 study it was the temperature that drove core heat.
My rule of thumb: if a listing promises heat several inches deep, read it as marketing; the probe data put most of the warming in the first 2 to 3 cm.
Bottom line: an infrared sauna warms muscle near the skin, and raises core temperature only when the cabin is hot, as at 65°C. For the health studies, see the benefits overview linked above.
Sources
- Reed EL, Uzoekwe CC, Atencio JK, Minson CT, Halliwill JR, “Muscle temperature increases during a single far infrared sauna session without changes in intestinal temperature,” Journal of Applied Physiology, 2025;138:1628-1637 (abstract): pubmed.ncbi.nlm.nih.gov
- Jenkins EJ et al., “Far-infrared sauna exposure at 65°C elevates core temperature,” Experimental Physiology, 2026: europepmc.org
- Mero A, Tornberg J, Mäntykoski M, Puurtinen R, “Effects of far-infrared sauna bathing on recovery from strength and endurance training sessions in men,” SpringerPlus, 2015;4:321: europepmc.org
- Hussain JN, Cohen MM, Mantri N, O’Malley CJ, Greaves RF, “Infrared sauna as exercise-mimetic? Physiological responses to infrared sauna vs exercise in healthy women: A randomized controlled crossover trial,” Complementary Therapies in Medicine, 2022;64:102798 (abstract): pubmed.ncbi.nlm.nih.gov