According to Deutsche Hochdruckliga (German High Pressure League):
The sauna session is initially a burden for the cardiovascular system. Skin and body temperatures can rise upto 40 degrees Celsius. There is an increase in the distribution of stress hormones, which also accelerates the heart rate continuously.
After 15 minutes of sauna, the heart rate values are the same as for a load of 90 watts on the bike ergometer – i.e. the effort when you run with a bag in your hand to the bus.
“Our studies have shown that the upper systolic value increases with the increasing length of stay in the sauna,” says Professor Ketelhut. “The lower diastolic pressure rises significantly at the beginning of the sauna, and then remains constantly increased.”
“The reduction in blood pressure only occurs in the resting time,” explains the expert.