## 6 Discussion  
In this study, we attempted to find the relationship between the perceived stress level declared in the questionnaires and the linear acceleration, angular velocity of head, electrooculographic signals, and heart rate registered by smart glasses and a finger pulse oximeter. The role of the finger pulse oximeter was providing the heart rate, and the smart glasses were used to acquire the linear acceleration and angular velocity of the subject's head and EOG signals, which may be used to capture eyeball movements and other activities [63]. The acquisition of heart rate and other signals to enhance the stress recognition accuracy was described by [PERSON] et al. in [3] and [PERSON] et al., in [6]. [PERSON] et al. in [3] achieved the stress level measurement accuracy of 87.5% by processing the EEG and ECG signals registered simultaneously. [PERSON] et al. in [6] presented an approach to recognize the stress in firefighters based on 24 h registrations of ECG and acceleration in combination with the results of self-assessment of stress level in questionnaires. The performance of their approach expressed by the area under the curve (AUC) index was between 0.57 and 0.7. Heart rate is the frequency of cardiac contractions and is susceptible to changes of the physical and mental condition. Therefore, heart rate is considered as one of the most important health indicators [58]. Many studies confirm the relationship between the HR and stress or anxiety level; [PERSON] et al. in [8] proved that the perceived heart rate is more significantly related to the level of anxiety than the measured heart rate. These findings play an important role in stress and anxiety coping strategies. In [9] [PERSON] et al. confirmed that work-related stress is strongly correlated with the heart rate and blood pressure; work-related stress in men was usually in association with higher blood pressure, whereas more prominent heart rate variability was usually observed in women. Moreover, stress also affects cognitive abilities; people suffering from stress make more mistakes and perform tasks less accurately [1]. In our study, the results of the [PERSON]'s test and post-hoc tests prove that the heart rate before and during the task was significantly higher than after the task (\(p<0.001\)), whereas the perceived stress level during performing the task was significantly higher than the stress level before and after the task (\(p<0.001\)). The [PERSON]'s rank correlation between the perceived stress level and heart rate was 0.561 (\(p=0.005\)), which could be considered as a significant correlation. These results prove the hypothesis that the stress level and heart rate are correlated. The differences between the power of total linear acceleration and total angular velocity before the task and during the task were statistically significant; however, we did not observe such differences for the power of the total EOG signal. This finding proves that the eyeball movements may not be correlated with the fact of performing the task, contrary to the findings of studies conducted by [PERSON] et al. [14; 76], [PERSON] et al. [77], [PERSON] et al. [78], and [PERSON] et al. [79]. The study conducted by [PERSON] et al. on a group of 50 students of physiology showed, as in our study, no statistically significant differences in the horizontal and vertical EOG values between the control group and the study group, classified in terms of visual dispersion. The authors emphasize that the visual distraction used in this study may not have been significant enough to deteriorate student performance in exams [19]. The literature presents a wide range of developed approaches to the selection of analyses for multidimensional data and various application purposes. Many existing methods, such as mean and variance values in sequence or first-order derivatives, are based on prior knowledge and manual examination, causing difficulties in providing detailed information and problems with statistical validation.