Nonlinear effects of winter swimming and sauna recreational activities on the heart rate variability

Sauna sessions and winter swimming are traditional and popular recreational activities in certain countries. Their positive effects on health and relaxation, both as separate and combined activities, are commonly reported. However, systematic studies of these effects are relatively scarce, especially regarding the nonlinear analysis of the physiological measurements of the heart activity. We performed Multi-Scale Entropy (MSE) and Detrended Fluctuation (DFA) analyses on the inter-beat time series (about 72 h long) of 21 healthy volunteers studied in three distinct contexts: winter swimming combined with sauna bathing (W), sauna bathing alone (S), and control (C) with no related activities. We confirmed that the scaling exponents (DFA) and complexity indices (determined from MSE) stay within the variation observed for healthy individuals as compared to public data sets. Next, we showed that S and W interventions have uncorrelated effects on the whole time series complexity in each individual. Additionally, the long-range scaling properties of S and W groups are not correlated as determined by DFA. Thus, we speculate that winter swimming combined with sauna bathing and sauna bathing alone might have different physiological responses.