When cedrol, an aroma compound found in wood essential oils, was released into a bedroom, the time taken to fall asleep was shorter and total sleep time was longer. A 2003 study reported these differences after recording the sleep of 11 young women throughout the night.
However, there were no significant differences in perceived sleep quality or sleepiness on waking, or in the proportion of deep sleep. A distinctive feature of this study was that it examined sleep recordings separately from participants’ own impressions, rather than relying only on reports of feeling better rested.
Testing a single, faint-smelling compound during sleep
Cedrol is a sesquiterpene alcohol found in oils such as cedarwood oil. The original paper explains that highly purified cedrol has a very weak odor, sometimes imperceptible even when the crystals are smelled directly.
Drawing on earlier reports about the autonomic nervous system, the researchers considered whether cedrol might ease the transition from wakefulness to sleep. Rather than using an essential oil containing a mixture of compounds, they aimed to test sleep itself using cedrol that had undergone repeated purification to reach 99.8% purity.
Eleven participants sleep with cedrol and without scent
The participants were 11 healthy female undergraduate and graduate students, with an average age of 24.5 years. All had some dissatisfaction with their sleep, but screening found that none had a sleep disorder.
First, they spent three days at home regulating their routines, including avoiding large changes in bedtime and waking time, with an activity monitor and sleep diary used to check this. They then stayed in the laboratory for six nights. The first two were adaptation nights to become accustomed to the environment, and the remaining four were used for the main experiment.
Each participant slept for two consecutive nights in the cedrol condition and two consecutive nights with no added scent. The order was counterbalanced: six experienced the unscented condition first, and five experienced cedrol first. Neither the participants nor the person scoring the sleep stages was told which condition was being used.
Cedrol was heated to release it into the air for four hours, from two hours before the participant’s usual bedtime until two hours afterward. Room temperature was kept at approximately 22°C and humidity at 50–60%. The scent was either imperceptible or only faintly noticeable.
Falling asleep about 7.5 minutes sooner, with about 13 more minutes of sleep
Brain waves, eye movements, chin muscle activity, and other signals were recorded, and sleep stages were scored in 20-second intervals. Total sleep time was significantly longer with cedrol: 408.0 minutes compared with 394.7 minutes in the unscented condition.
Sleep-onset latency, the time from lights out to falling asleep, was significantly shorter, decreasing from 16.8 to 9.3 minutes. On average, this amounts to falling asleep about 7.5 minutes sooner and sleeping about 13.3 minutes longer.
Sleep efficiency, the proportion of time in bed actually spent asleep, was higher numerically, rising from 91.9% to 95.2%. However, the difference had a p value of 0.08 and was not significant at the conventional 5% threshold. Although one passage in the original discussion summarizes sleep efficiency as having increased, the result needs to be distinguished as a trend toward an increase.
What about waking during the night and deep sleep?
Total time awake during the night was 11.8 minutes without scent and 5.3 minutes with cedrol, but the difference was not significant. When only awakenings lasting at least one minute were considered, the duration was lower, from 7.8 to 1.7 minutes, but this too was a trend with p = 0.08.
An analysis dividing sleep into first and second halves found a trend toward less time in awakenings lasting at least one minute during the first half. Meanwhile, neither the proportion of deep, slow-wave sleep nor the proportion of REM sleep differed significantly. The findings for sleep onset and total sleep time cannot be extended to suggest that every aspect of sleep improved.
Participants’ ratings of their sleep and sleepiness on waking also did not differ between conditions. For relaxation before bed, the paper reports that all 11 participants chose the very-relaxed response in the cedrol condition, but it does not report a test of the between-condition difference for this item. The findings show that differences in sleep recordings and people’s own perceptions do not necessarily move together.
The autonomic explanation was not directly tested here
The authors discuss the possibility that cedrol suppressed sympathetic nervous system activation and helped a shift toward parasympathetic predominance, making it easier to fall asleep. However, although the paper says that heart rate, respiration, and other signals were recorded, it does not present the results of those autonomic analyses. The explanation of the pathway is mainly an inference based on previous research.
The authors also discuss the possibility that heat loss and a reduction in core body temperature were involved, but core temperature was not measured. The proposed advantage that a faint scent would be less affected by liking or dislike is also the authors’ interpretation, not the finding of a trial comparing people with different preferences.
What was tested here was a short period of sleep in a controlled laboratory among 11 young women without sleep disorders. Commercial essential oils are mixtures of compounds, and concentrations and methods of releasing them into the air at home also differ. The original paper looks toward future applications in sleep environments, but these results alone do not establish effects of essential-oil use at home or benefits for insomnia.
Summary
When cedrol was released into the air for four hours beginning before bedtime, sleep-onset latency was shorter than in the unscented condition, decreasing from 16.8 to 9.3 minutes, and total sleep time was longer, rising from 394.7 to 408.0 minutes. Sleep efficiency and some measures of nighttime awakening showed only trends toward improvement, while the proportions of deep and REM sleep and subjective ratings on waking did not differ significantly.
Using a single faint-smelling compound and overnight sleep recordings rather than participants’ impressions alone, the study found differences in sleep onset and sleep duration. Those two findings are its core results. The mechanism and reproducibility in broader populations and everyday settings remain questions for further research.
Research context and disclosures: At the time of the study, seven of the eight authors were affiliated with Kao Corporation’s Tokyo Laboratories, and one with the National Institute of Mental Health, National Center of Neurology and Psychiatry. No separate funding or conflict-of-interest statement could be identified in the original paper.
References
Yamamoto, Y., Shirakawa, S., Nagashima, Y., Ohsu, H., Tojo, S., Suzuki, M., Yada, Y., and Suzuki, T. (2003). The Effects of Cedrol on Sleep. Japanese Journal of Physiological Anthropology, 8(2), 69–73. Original study / Open PDF
