## 2 Material and Methods
### Study Procedure  
Three measurements were taken in the study: measurement 1, carried out 24 h before cold-water immersion (winter swimming) (blood collection on 18 February 2023, psychological questionnaires); measurement 2, carried out only in the group of winter swimmers, immediately after coming out of the 4 \({}^{\circ}\)C water, with an air temperature of 5 \({}^{\circ}\)C (pressure 1011 hPa, wind speed 25 km/h, humidity 90%, light rain showers on that day; psychological questionnaires); measurement 3, carried out 24 h after cold-water immersion (winter swimming) (blood collection on 20 February 2023, psychological questionnaires). In order to eliminate uncontrolled variables (weather conditions, air pressure, etc.), measurements 1 and 3 were performed over the same period in both the control and experimental groups.  
Fasting blood was collected twice from all subjects, from an ulnar vein into vacuum tubes, in the middle of the 2022/2023 winter swimming season, 24 h before cold-water immersion (winter swimming) and 24 h after the immersion. The blood was collected in 10-mL Vacuette tubes by a qualified nurse, under a physician's supervision, in accordance with the applicable standards. At the same time, the control group participants were subjected to the same testing procedure (without contact with cold water). Serum blood indicators were determined in the Diagnostyka S.A. laboratory in Krakow, Poland. Adrenaline (pmol/L), noradrenaline (pmol/L), and serotonin (ng/mL) were assessed with the use of the ELISA method (test by LDN, Nordhorn, Germany). Dopamine (pmol/L) was identified with the HPLC method (Agilent, Santa Clara, CA, USA). Cortisol (ug/dL) was determined with chemiluminescence (Abbott, Abbott Park, IL, USA). The subjects were also  
\begin{table}
\begin{tabular}{l l l l l l l l l} \hline  & & & & \multicolumn{3}{c}{**Percentiles**} & & \multicolumn{3}{c}{**Mann–Whitney _U_**} \\ \hline  & **Group** & **Mean** & **SD** & **25 th** & **50 th** & **75 th** & \(U\) & \(p\) \\ \hline Age (years) & Control & 46 & 4.19 & 43 & 45.5 & 48 & 98.0 & 0.574 \\  & Winter swimmers & 43.5 & 7.77 & 3.5 & 46 & 49.25 & & \\ Physical activity (hours & Control & 5.31 & 3.28 & 3.75 & 4.5 & 7.25 & 108.5 & 0.469 \\ per week) & Winter swimmers & 6.94 & 4.77 & 3.75 & 5 & 9 & & \\ Experience in winter & Control & – & – & – & – & – & – & – \\ swimming (years) & Winter swimmers & 4.84 & 3.98 & 2 & 3.5 & 7.25 & & \\  & Control & 88.07 & 16.33 & 75.5 & 86 & 98 & 85.5 & 0.279 \\ Body mass (kg) & Winter swimmers & 83.25 & 15.45 & 72 & 80 & 89.25 & & \\  & Control & 179.21 & 5.13 & 175 & 179 & 184 & 107.0 & 0.850 \\ Body height (cm) & Winter swimmers & 178.63 & 4.95 & 176 & 177 & 180.25 & & \\ \hline \end{tabular}
\end{table}
Table 1: Statistics characterizing the study groups.  
asked to complete a battery of psychological tests consisting of the following questionnaires: the Profile of Mood States (POMS) questionnaire [20] in the Polish adaptation by [PERSON] and [PERSON] [21], and the Satisfaction with Life Scale (SWLS) by [PERSON] et al. (1985) [22] in the Polish adaptation by [PERSON] [23].  
The POMS questionnaire is a self-description tool consisting of 65 items to which the respondent is asked to refer on a 5-point Likert scale (0-4). This questionnaire measures the intensity of 6 emotional states: tension-anxiety, depression-dejection, anger-hostility, fatigue, confusion, and vigor. The measurement can refer to any specified period. In the case of this research, the respondents were asked to indicate their state 'here and now'.  
The SWLS scale is a 5-item self-report inventory (on a 7-point scale) that measures satisfaction with life [23].  
The procedure for the conducted tests is shown in Figure 1.