Short read
Juhani Leppäluoto and colleagues at the University of Oulu tested eight healthy young men under three heat conditions, published in the European Journal of Applied Physiology in 1986. The conditions were 80 degrees dry until subjective discomfort, 100 degrees dry until subjective discomfort, and 80 degrees with rising humidity until subjective exhaustion. Each condition was compared with the same men's starting values; there was no separate control group.
Measured against baseline, oral temperature rose by 1.1, 1.9 and 3.2 degrees across the three conditions, and heart rate by about 60, 90 and 130 %. Noradrenaline rose by about 100, 160 and 310 %, and prolactin two-fold, seven-fold and ten-fold. Adrenaline did not change in any condition, which argues against a simple stress response. Temperature and humidity shaped the response more than the length of exposure did.
Eight men and a study from 1986 make a small and dated basis. The highest values come from the condition carried to subjective exhaustion, which is a study protocol and explicitly not a pattern of use. The measurements are acute and say nothing about adaptation over time.
The value of the study is its gradient. It shows sauna heat as a graded physiological load rather than a pause, with the response rising as temperature and humidity rise. It does not describe what an ordinary session does to an individual, and it gives no basis for seeking exhaustion in the heat.