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Laukkanen and colleagues at the University of Eastern Finland measured what happens in the body during and directly after a single sauna session, published in the Journal of Human Hypertension in 2018. The participants had at least one cardiovascular risk factor. Blood pressure, heart rate and arterial stiffness were recorded before, during and after the session, so each person served as their own comparison.
After a 30-minute session, systolic and diastolic blood pressure were lower than before it. Pulse wave velocity, which reflects how stiff the arteries are, fell on average from 9.8 to 8.6 metres a second compared with the same participants' values before the session.
The measurements are short-term and say nothing about how long the changes last. There is no control group in the classic sense, only the comparison with each participant's own starting values. The study therefore describes an acute response, not a lasting adaptation, and it does not measure any health outcome.
Its value lies in the mechanism. The large Finnish cohorts report associations between frequent sauna use and cardiovascular outcomes over decades; this study shows which acute changes in blood pressure and vessels occur in a single session. Whether repeated acute responses of this kind explain the cohort findings is not answered by these data.