## 3 Results  
### Analysis of HRV Parameters  
HRV analysis was conducted both during the cognitive task and in a state of relative rest. Results showed that during the letter cancellation test before light exposure, there was an increase in HR, AMo, SIM, and IB, and a decrease in SDNN, RMSSD, HRVind, DX, PAR, and Mode compared to baseline conditions (\(p<0.05\)) (Table 4). However, after the Blue Sky Pro session and during the letter cancellation test, the values of the above parameters (HR, AMo, SDNN, RMSSD, HRVind, DX, PAR, and Mode), except for SIM and IB, did not differ from the baseline values (\(p>0.08\)).  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Parameter**} & \multicolumn{5}{c}{**Condition 1** } & \multicolumn{5}{c}{**Comparison 2 (p0)**} \\ \cline{2-9}  & **Fon** & **Cor** & **Blue1** & **Blue2** & **BlueCor** & **Fon vs.** & **Fon vs.** & **Cor vs.** \\  & & & & & & & & \\ \hline
**SDNN, ms** & 61.6 \(\pm\)2.9 & 55.1 \(\pm\)3.7 & 61.7 \(\pm\)4.6 & 62.4 \(\pm\)3.4 & 56.9 \(\pm\)3.3 & 0.049 & 0.136 & 0.379 \\ \hline
**RMSSD, ms** & 50 & 38 & 48 & 45 & 42 & & & \\  & (38.60) & (29.54) & (36.63) & (38.67) & (36.69) & 0.044 & 0.177 & 0.044 \\ \hline
**NN50, ms** & 86 & 49 & 92 & 81 & 57 & & & \\  
When comparing HRV during the letter cancellation test before and after the Blue Sky Pro session, the following trends were observed (Figure 1): after the Blue Sky Pro session, HR values decreased by 4.5 \(\pm\) 1.1 beats/min (_p_ = 0.001), SIM decreased by 0.6 \(\pm\) 0.3 c.u. (_p_ = 0.049), IB decreased by 24 \(\pm\) 8 c.u. (_p_ = 0.028), and the values of Mode and RMSSD increased by 43 \(\pm\) 11 ms (_p_ = 0.002) and 5.3 \(\pm\) 2.5 ms (_p_ = 0.044), respectively.  
The HRV spectral analysis results during the letter cancellation test, both before and after light exposure, showed a decrease in **LF**, **HF**, and **Total** compared to the baseline condition (_p_ < 0.05). However, a comparative analysis of HRV spectrogram parameters during cognitive tasks before and after the Blue Sky Pro session did not reveal any statistically significant differences (_p_ > 0.3). In the resting state during the Blue Sky Pro session, the intragroup HRV analysis showed the following trends: during the first 5 min of light exposure (Blue1), **HR** and **HF** values decreased by 1.5 \(\pm\) 0.7 beats/min and 753 \(\pm\) 218 ms\({}^{2}\), respectively, compared to the baseline, while **Mode** increased by 20 \(\pm\) 9 (_p_ < 0.05) (Figure 2).  
Figure 1: HRV parameters during the cognitive task before (Cor) and after the Blue Sky session (BlueCor): (**a**) HR, bpmm—heart rate; (**b**) Moda, ms—NN-interval duration mode; (**c**) SIM, c.u.— intensity index of influences of the sympathetic division of the autonomic nervous system; (**d**) RMSSD, ms—the square root of the sum of the squared differences in the durations of consecutive NN-intervals pairs; (**e**) IB, c.u.—stress index of regulatory systems according to [PERSON]. [PERSON].  
Figure 2: HRV parameters reflecting statistically significant differences between the study stages: (**a**) HR, bpmm—heart rate; (**b**) Moda, ms—NN-interval duration mode; (**c**) HF, thousand ms\({}^{2}\)—the power of high-frequency (0.15–0.40 Hz) oscillations in the NN-interval duration.  
The aforementioned patterns depict a decline in the intensity of sympathetic nervous system influences at the start of the Blue Sky Pro session. Examining HRV during the last five minutes of the 20-min light exposure (Blue2) did not exhibit any statistically significant changes in HRV in comparison to the baseline (\(p>0.09\)).