Regular postexercise sauna bathing does not improve heart rate variability: A multi-arm randomized controlled trial.

Regular exercise has been shown to increase heart rate variability (HRV) for different populations. Acute and short-term studies using heat therapy and sauna bathing have also shown HRV improvements. However, long-term adaptations in HRV to regular exercise and sauna bathing remain unexplored. In this 1:1:1 multi-arm trial, sedentary participants (n = 38) aged 49 ± 9 years with at least one CVD risk factor were randomly assigned to regular exercise and 15-min postexercise sauna (EXS), regular exercise only (EXE), or control (CON) group, for an 8-week intervention. Indices of HRV (RR interval, RMSSD, SDNN, resting heart rate [HR], HRMAX-HRMIN, high frequency power [HFP], and low frequency power [LFP]) were measured before (PRE) and after (POST) the trial. Compared to CON, EXE increased the time-domain measure of HRMAX-HRMIN (p = 0.003), and elicited significantly smaller decreases in the frequency-domain measure of LFP (p = 0.022). There were no statistically significant differences between EXS and EXE for any of the HRV indices measured. Eight weeks of regular exercise conferred positive changes in both time- and frequency-domain measures of HRV. However, adding regular sauna bathing postexercise offered no additional benefits to HRV over regular exercise alone.