Objective:
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.
Sources:
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