To assess whether nighttime inhalation of Kagawa Hinoki root essential oil affects sleep in middle-aged women.
Approach:
Design and participants: Single-blind crossover study of 12 Japanese women aged 35–60 years. Participants completed two 2-week conditions—Hinoki root oil and unscented squalane—separated by a 1-week washout.
Exposure and assessment: Before bed, participants inhaled the assigned oil for 10 seconds and kept the cup near the bed. Sleep was assessed with a questionnaire, wearable EEG, and smartwatch measurements.
Key Findings:
After 13 days, nighttime wakefulness was lower with Hinoki root oil than with control; smartwatch data from 10 participants showed a statistically significant difference and an interaction between condition and measurement period.
After 13 days, participants reported better sleepiness upon awakening and fatigue recovery with the oil. Sleep maintenance, dreaming, and perceived sleep duration did not differ significantly between conditions.
EEG showed a greater increase in relative delta-wave activity during the first 20 minutes after lying down with the oil than with control after 7 days.
There were no significant between-condition differences in overall smartwatch sleep scores, total sleep duration, sleep efficiency, or estimated deep sleep.
Alpha-pinene was the most abundant measured emitted volatile, accounting for 37% of volatile peak area; the study did not establish effects of individual compounds.
Interpretation:
The authors suggested that Hinoki root volatiles may facilitate the transition toward deeper sleep, while noting that observed improvements were limited to selected sleep measures.
Limitations:
Small sample of 12 participants, with some outcomes available for fewer participants because of recording errors.
Incomplete physiologic recordings and use of wearable devices rather than polysomnography.
Bedroom temperature and humidity were not analyzed, and BMI data were not collected.
Findings were limited to middle-aged Japanese women.
Conclusion:
The study authors described the findings as a foundation for further research. They declared no conflicts of interest.
