Background: In recent years, increasing attention has been paid to the physiological effects of nature-derived stimulation. The physiological relaxation effects caused by forest-derived olfactory stimuli have been demonstrated. However, there are no studies on the physiological effects of olfactory stimuli by Hinoki cypress (Chamaecyparis obtusa) leaves. We investigated the effects of olfactory stimulation by Hinoki cypress leaf oil on the left/right prefrontal cortex activity, assessed using near-infrared time-resolved spectroscopy (TRS), and on the autonomic nervous activity, assessed by measuring heart rate variability (HRV).
Method: Thirteen female university students (mean age, 21.5 ± 1.0 years) participated in the study. Physiological measurements were performed in an artificial climate maintained at 25 °C, 50% relative humidity, and 230-lx illumination. Hinoki cypress leaf oil was used as an olfactory stimulation with air as the control. The odor was administered for 90 s, while the subjects sat with their eyes closed. Oxyhemoglobin (oxy-Hb) concentrations were measured in the prefrontal cortex using TRS. The high-frequency (HF) component of HRV, which is an estimate of parasympathetic nervous activity, and the low-frequency (LF)/(LF + HF) ratio, which is an estimate of sympathetic nervous activity, were measured by electrocardiography. A modified semantic differential method was used to perform subjective evaluations.
Results: Olfactory stimulation by Hinoki cypress leaf oil induced a significant reduction in oxy-Hb concentration in the right prefrontal cortex and increased parasympathetic nervous activity. The subjects reported feeling more comfortable.
Conclusion: These findings indicate that olfactory stimulation by Hinoki cypress leaf oil induces physiological relaxation.