Physiological adjustment effects of viewing natural environment images on heart rate variability in individuals with depressive and anxiety disorders

Nature-based interventions improve mood and reduce stress, but their effects on heart rate variability (HRV) remain inconsistent. The physiological adjustment effect theory proposes HRV adjusts to an optimal value during nature contact-increasing when baseline HRV is low and decreasing when high. This study tested this theory in patients with depressive and anxiety disorders and investigated its association with mood changes. A randomized crossover trial was conducted with 60 patients. We assessed the correlation between baseline HRV during control images and HRV change after 3 min natural image viewing. We also examined mood changes in comfort, relaxation, and vigor. A significant negative correlation was found between baseline low-frequency/high-frequency (LF/HF) ratio and changes after viewing natural images, indicating a physiological adjustment effect in sympathetic activity. No significant correlation was found with the parasympathetic indicator, HF. Participants with adjustment effects in LF/HF had greater improvement in vigor. Those without these effects had higher alcohol use and tended towards more comorbid hypertension. Natural interventions may enhance vigor through physiological adjustment in sympathetic activity. The lack of adjustment effects is linked to behaviors and conditions associated with autonomic dysfunction, highlighting the need for targeted therapies in depressive and anxiety disorders.