The published outcomes are different

The large cohorts measured mortality and disease. The acute heat study measured heart rate and blood pressure during and after one session. The randomized post-exercise trial measured blood pressure, cholesterol, and cardiorespiratory fitness.

None of the studies in this library measured HRV. Claiming that they proved an HRV improvement would replace the measured outcome with a wearable metric the researchers did not test.

Your wearable can answer the personal question

Nobody has run the trial on your body. Hold the sauna schedule steady, compare the same-day and next-day readings with your pre-experiment baseline, and look for a repeatable direction rather than one unusually good morning.

HRV and resting heart rate are useful here precisely because they are measured regularly. They let you ask whether your own response matches the broader cardiovascular rationale without pretending they are the published endpoint.

What the research says

Ketelhut and Ketelhut measured blood pressure and heart rate in 19 healthy adults during a 25-minute sauna at 93C (199F) and 13% humidity. Their Complementary Therapies in Medicine study from 2019 found a response comparable to a 60–100 W exercise workload during the session, followed by blood pressure below baseline afterward. It did not measure HRV or resting heart rate adaptation.

Lee et al. randomized 47 participants across exercise plus sauna, exercise alone, and control for 8 weeks. Their American Journal of Physiology-Regulatory, Integrative and Comparative Physiology trial from 2022 found that adding a 15-minute post-exercise sauna produced about an 8 mmHg larger systolic blood pressure reduction and a larger total cholesterol reduction than exercise alone. Exercise alone produced the larger fitness gain, about 6.2 mL/kg/min versus about 2.7 mL/kg/min for exercise plus sauna relative to control. The trial did not measure HRV.

Kunutsor et al. measured the joint association of cardiorespiratory fitness and sauna use with cardiovascular and all-cause mortality in a long-term prospective cohort. Their Annals of Medicine study from 2018 found that high fitness plus frequent sauna was associated with a larger risk reduction than either one alone. The association does not establish cause, and the study did not measure HRV.

Laukkanen, Laukkanen, and Kunutsor's Mayo Clinic Proceedings review from 2018 summarized cardiovascular and other sauna evidence rather than enrolling a new participant population. It supports a cardiovascular rationale for study, not a claim that sauna raises HRV.