## 3 Results and Discussion
### HRV Analysis  
HRV refers to the variation in time intervals between consecutive heartbeats and is recognized as a key physiological indicator that indirectly reflects activity changes in the autonomic nervous system, including both sympathetic and parasympathetic systems [20]. In this study, the HRV of participants exposed to wooden and non-wooden environments was measured simultaneously. For a comparative analysis, the non-wooden environment and the wooden environments (45% and 90%) were grouped for evaluation. The results showed no statistically significant pre-post changes in any HRV indicators for participants exposed to the non-wooden environment. The 45% wooden environments group showed statistically (\(p<0.05\)) significant changes in HRV indicators such as SDNN, lnHF, and SI pre- and post-exposure (Table 3). SDNN increased from 28.85 \(\pm\) 9.02 pre-exposure to 41.78 \(\pm\) 16.12 after exposure. An increase in SDNN is known to be associated with enhanced parasympathetic nervous system activity, reflecting a physiological state of relaxation [21]. lnHF increased from 4.77 \(\pm\) 1.42 to 5.31 \(\pm\) 1.10 following exposure to the wooden environment. In the frequency domain analysis, the HF component (0.15-0.4 Hz) reflects parasympathetic nervous system activity and is generally associated with relaxation or rest states [22]. SI is a stress index associated with the pressure exerted on the heart. In the wooden environment, SI decreased from 2.31 \(\pm\) 1.77 to 1.45 \(\pm\) 0.75 after exposure. The observed increase in SDNN and lnHF values, along with a decrease in SI, suggests enhanced parasympathetic nervous system activity and a corresponding relaxation response in the 45% wooden environment. Several previous studies have reported changes in HRV among participants in wood-based interior environments. [PERSON] et al. measured the HRV of patients by installing a pine wood wall and arch wood seats in the waiting area of the hospital. The LF/HF ratio decreased by 26.4% in a wood-finished waiting room, indicating autonomic stabilization and relaxation effects [22]. The LF/HF ratio is used to assess the balance between sympathetic and parasympathetic nervous activity, with lower values indicating a more relaxed state. [PERSON] and [PERSON] reported that SDNN, RMSSD, and LF/HF values tended to decrease after contact with wood. They suggest that contact with wood promotes parasympathetic activation and overall relaxation, shifting the autonomic balance toward parasympathetic dominance [23]. However, parasympathetic dominance in wooden environments does not always appear consistently, and some studies have reported opposing results. [PERSON] et al. observed a gradual increase in heart rate during short-term exposure to wooden environments, attributing this to a \"wow effect\" associated with initial wood exposure [24]. On the other hand, although variations in the indicators were identified under the a 90% wooden environment, no statistically significant differences were found. The absence of significant changes in HRV under both the 0% and 90% wood coverage conditions may be attributed to different underlying mechanisms. In the 0% condition, the absence of natural material stimuli fails to induce physiological relaxation. Conversely, in the 90% condition, a saturation effect or psychological adaptation may occur, where excessive visual exposure to wood no longer enhances and may even diminish restorative responses. This phenomenon has been documented in previous studies on restorative environments, which indicate that including additional natural elements beyond an optimal level does not yield further benefits [25]. [PERSON] et al. created three test rooms with wood coverage rates of 0%, 45%, and 90%, and randomly exposed 15 participants to each room for 90 s. They found that the participants' physiological relaxation effects did not necessary increase with a higher wood coverage.