Nighttime exposure to KagawaHinoki root essential oil may reduce time spent awake during the night and improve morning sleepiness and fatigue recovery in middle-aged women.
The researchers conducted a single-blind crossover study involving 12 Japanese women aged 35 to 60 yearswho worked daytime shifts, were nonsmokers, and had no reported psychiatric disorders, neurologic treatment, or olfactory abnormalities. Each participant completed 2 experimental conditions lasting 2 weeks each, separated by a 1-week washout period. Before bedtime, the participants inhaled about 3 drops of Hinoki root essential oil or unscented squalane oil from a paper cup for 10 seconds, then positioned the cup about 30 cm from their nose while sleeping.
Sleep quality was assessed using the Oguri-Shirakawa-Azumi Sleep Inventory, middle-aged and aged version, alongside wearable electroencephalography and smartwatch measurements. The questionnaire evaluated sleepiness upon awakening, sleep maintenance, dreaming, fatigue recovery, and perceived sleep duration. Physiologic assessments included brain-wave activity during the first 20 minutes after lying down, nighttime wakefulness, sleep efficiency, total sleep duration, and estimated deep sleep.
Following 13 days of exposure, the participants using Hinoki root essential oil spent less time awake during the night compared with the unscented control. Smartwatch data from 10 participants showed a statistically significant difference in nighttime wakefulness, with a significant interaction between the exposure condition and measurement period. The difference in nighttime wakefulness varied across the exposure period.
Self-reported sleep outcomes also favored Hinoki root essential oil following 13 days. The participants reported improved sleepiness upon awakening and fatigue recovery compared with the control condition. Across measurement periods, the researchers identified statistically significant differences in both outcomes. However, other questionnaire measures, including sleep maintenance, dreaming, and perceived sleep duration, did not show statistically significant differences between conditions.
Electroencephalographic measurements showed changes in brain-wave activity during the transition from wakefulness to sleep. Following 7 days of exposure, the increase in relative delta-wave activity between the first and second 10-minute periods after lying down was greater with Hinoki root essential oil vs. control. Within the essential oil condition, delta-wave activity increased following 7 and 13 days, accompanied by changes in theta- and beta-wave activity. The researchers suggested exposure to Hinoki root volatiles may facilitate the transition toward deeper sleep.
Despite these findings, smartwatch measurements showed no statistically significant differences in overall sleep scores, total sleep duration, sleep efficiency, or estimated deep sleep between conditions. Sleep scores were available for 8 participants because of recording errors, while other smartwatch outcomes were evaluated in 10 participants. Improvements were limited to selected aspects of sleep.
The participants rated the essential oil more favorably than the control for relaxation, comfort, scent liking, and several other aroma characteristics. Chemical analysis identified alpha-pinene as the most abundant emitted volatile compound, accounting for 37% of the measured volatile peak area, followed by delta-cadinene, gamma-cadinene, limonene, and alpha-muurolene. The researchers discussed the potential contribution of these compounds to sleep-related effects, although they did not establish the role of individual constituents.
The study was limited by its small sample size, incomplete physiologic recordings, and reliance on wearable sleep-monitoring devices rather than polysomnography. Environmental factors, including bedroom temperature and humidity, were not incorporated into the analysis, and body mass index data were not collected. The findings were also limited to middle-aged Japanese women, restricting their generalizability to other populations.
“Despite these limitations, the present findings provide a valuable foundation,” wrote lead study author Fadilla Zennifa, of the Department of Agri-Environmental Sciences at Kyushu University, and colleagues.
The study authors declared no conflicts of interest.
Source: Journal of Wood Science
