## 2 Materials and Methods
\begin{table}
\begin{tabular}{c l} \hline \hline
**Parameter** & **Definition** \\ \hline
**SDNN, ms** & Standard deviation of the NN-interval duration for the sample \\ \hline
**RMSSD, ms** & The square root of the sum of the squared differences in the durations of \\  & consecutive NN-intervals pairs \\ \hline
**pNN50, \%** & Percentage of the adjacent NN-intervals pairs that differ in duration by more \\  & & than 50 ms \\ \hline
**HRVind, c.u.** & Triangular index of heart rate variability \\ \hline
**Heart rate, beats/min** & Heart rate \\ \hline
**Mode, ms** & NN-interval duration mode \\ \hline
**AMo, \%** & Amplitude of the NN-interval duration mode \\ \hline
**DX, ms** & Variation range of NN-interval duration \\ \hline
**VLE thousand ms\({}^{2}\)** & The power of very low-frequency (0.003–0.04 Hz) fluctuations in the duration of NN-interval \\ \hline
**LF, thousand ms\({}^{2}\)** & The power of low-frequency (0.04–0.15 Hz) fluctuations in the duration of NN-interval \\ \hline
**HE, ms\({}^{2}\)** & The power of high-frequency (0.15–0.40 Hz) fluctuations in the duration \\  & NN-interval \\ \hline
**Total, thousand ms\({}^{2}\)** & The power of fluctuations in the duration of NN-interval in all ranges \\  & (0.003–0.40 Hz) \\ \hline
**LF norm, \%** & The normalized value of the power of low-frequency (0.04–0.15 Hz) fluctuations in the NN-interval duration \\ \hline
**HF norm, \%** & The normalized value of the power of high-frequency (0.15–0.40 Hz) fluctuations in the NN-interval duration \\ \hline
**LF/HF** & The ratio of low-frequency (0.04–0.15 Hz) and high-frequency (0.15–0.40 Hz) \\  & fluctuations in the NN-interval duration \\ \hline
**SIM, c.u.** & Intensity index of influences of the sympathetic division of the autonomic nervous system \\ \hline
**PAR, c.u.** & Intensity index of influences of the parasympathetic division of the autonomic nervous system \\ \hline
**IB, c.u.** & Stress index of regulatory systems according to [PERSON] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Examined parameters of HRV. The participants in the experiment performed the Bourdon test, also known as the letter cancellation test, online using a desktop computer. The test was administered twice, for a duration of 5 min each time, before and after the blue light exposure. This test is used to assess the level of attention, fatigue, performance, and resistance to monotonous activities that require a high level of sustained attention. The test involves tables consisting of 40 rows of letters with 40 letters in each row. The subject is required to sequentially search for and cross out the letters that appear first in each row, starting from the top row and proceeding from left to right, within the allotted time of 5 min. The limited time for performing the cognitive task contributes to stress induction. When analyzing the results of the letter cancellation test, 10 parameters were studied (Table 2). The experiments were conducted during the daylight hours (12:00-16:00). Prior to the study, the subjects were instructed to rest for 10-15 min. During the heart rate variability (HRV) registration, the subjects remained in a seated position with their eyes open. HRV was measured using photoplethysmography from the ring finger on the left hand, utilizing the ELOKS-01 pulse oximeter. HRV was recorded for 5 min during the resting state (baseline condition, Fon), for the initial 5 min of the Blue Sky Pro session (**Blue1**), and from the 15 th to the 20 th minute of the Blue Sky Pro session (**Blue2**). HRV registration was also performed during the letter cancellation test before (**Cor**) and after a 20-min Blue Sky Pro session (**BlueCor**) (Table 3). Participants were instructed to sleep for at least 7 h on the eve of the experiment, and it was also prohibited to consume caffeinated beverages, alcohol, and drugs the day before the study.