A small study from Japan provides interesting evidence that the scent of hinoki roots may influence sleep. Women who used the essential oil in their bedrooms for two weeks woke up less often at night and felt less tired in the morning. Brain activity while falling asleep also changed. The results are interesting—but, given the very small number of participants, they still need to be confirmed in larger studies. The findings were published in the Journal of Wood Science.

When Sleep Is No Longer a Given

Sleep problems can become more common in middle age. Many factors play a role here: work-related pressures, family obligations, changes in daily routines, stress, or physical changes. Sleep is not a state in which the brain simply shuts down. Throughout the night, the activity of the nervous system is constantly changing. The body goes through various sleep stages that repeat in a specific rhythm. The body’s response to sounds, light, smells, and other environmental stimuli also changes throughout the night. This is precisely why sleep research is increasingly focusing on how the bedroom environment influences sleep.

A new study from Japan focuses on an unusual factor: the scent of the Japanese cypress Hinoki (Chamaecyparis obtusa). Researchers at Kyushu University investigated whether an essential oil derived from the roots of this tree can influence sleep quality under real-world conditions at home. The Japanese research team was particularly interested in middle-aged women. Instead of studying the effect of the scent for just a single night in a laboratory, the scientists wanted to know how repeated exposure in a normal bedroom would affect sleep.

This is highly relevant to sleep research. After all, sleep under laboratory conditions is not necessarily the same as sleep at home. In one’s own bed, everyday habits, room temperature, noise, light, and personal daily routines all influence a good night’s rest. The researchers therefore opted for an experiment that would take place as closely as possible to the participants’ normal daily lives.

What Makes Hinoki So Special

Hinoki is a species of cypress common in Japan and is traditionally used, among other things, as high-quality timber. However, when the wood is harvested, parts of the tree—including the roots—are often left unused. It is precisely these roots that contain aromatic compounds from which essential oils can be extracted. To date, scientific research has focused more on the properties of essential oils derived from hinoki leaves and twigs. The oil from the roots, on the other hand, has been studied much less extensively.

The current study therefore has two distinct objectives: First, the researchers wanted to determine whether a valuable product could be extracted from the roots, which have been little used to date. Second, they sought to investigate whether the aromatic compounds contained in the roots might influence sleep.

An Experiment Right in the Bedroom

Twelve Japanese women between the ages of 35 and 60 participated in the pilot study. The average age was about 46. The study was conducted as a single-blind crossover study. Each participant used both the hinoki root oil and an odorless control product. Both phases lasted two weeks each. Before going to bed, the participants placed three drops of the respective product into a small paper cup. They then inhaled the scent for ten seconds. Afterward, the cup was placed about 30 centimeters away from their heads and remained there throughout the night. Unscented squalane oil served as the control.

The experimental design was thus deliberately simple. The researchers specifically did not want to investigate the effects of intensive aromatherapy under controlled laboratory conditions. Instead, the aim was to test whether a potential effect would also manifest during straightforward use in the participants’ own bedrooms. It is interesting to note that the scientists did not rely exclusively on the participants’ subjective assessments. Three different methods were used. Brain activity was recorded using an EEG headband. In addition, smartwatches monitored various sleep parameters. Every morning, the women also completed a questionnaire about their sleep and how they felt upon waking.

This allowed the researchers to combine subjective perceptions with physiological measurements. This is particularly important in sleep studies. For example, a person may feel as though they slept poorly, even though certain objective sleep parameters have hardly changed—or vice versa. Combining different measurement methods can therefore provide a more nuanced picture.

After One Week, Brain Activity Changed, After Nearly Two Weeks, the Women Woke Up Less Frequently

One of the most interesting observations emerged after just seven days. Under the hinoki condition, delta wave activity increased during the early sleep phase among the participants. Delta activity is primarily associated with slower brain waves during deep sleep. The researchers interpret the observed change as an indication that the transition to deeper sleep stages may be facilitated by the scent.

However, caution is warranted. A change in delta activity does not automatically mean that a person sleeps longer or better overall. It primarily indicates that the brain’s electrical activity has changed during the sleep phase in question. This distinction is particularly important in chronobiology: sleep quality cannot be assessed based on a single measurement. Another effect became apparent later in the study period.

On the 13th day of use, the smartwatches recorded fewer instances of waking up during the night. At the same time, the participants reported feeling less sleepy upon waking and having recovered better from fatigue than during the control phase. This revealed an interesting temporal pattern: After about a week, a change in brain activity was observed during the onset of sleep. After just under two weeks, there were also indications of fewer nighttime awakenings and improved subjective well-being in the morning. However, it is not yet possible to conclude from this whether a long-term adaptation of the sleep system is actually taking place. A significantly larger and longer study would be necessary for that.

What the Study on Sleep at Home Shows

The perception of the scent was also examined. Compared to the control condition, the participants rated the hinoki aroma as, among other things, more relaxing, pleasant, appealing, elegant, nostalgic, and having a stronger woody scent. This point is not insignificant. Scents can be closely linked to emotions, memories, and the subjective sensation of relaxation. It is therefore entirely conceivable that not only individual chemical components of the essential oil play a role, but also the psychological effect of the pleasant scent. However, the study cannot definitively determine which mechanism is responsible for the observed changes.

The researchers also analyzed the volatile components of the essential oil. Among the most frequently detected compounds were α-pinene, δ-cadinene, γ-cadinene, limonene, and α-muurole. This chemical composition is interesting because essential oils are not single active ingredients in the traditional sense, but rather complex mixtures of various volatile compounds. This raises an important question for further research: Is it actually the entire fragrance profile of hinoki oil that produces the observed effects—or are individual components responsible? The current study cannot yet answer this question.

One of the study’s strengths lies precisely in its real-life setting. The participants did not have to go to a special sleep environment. They used the oil in their own bedrooms and continued with their normal daily routines. In this way, the study addresses a question that is more relevant to many people than a perfect laboratory experiment: Can a relatively simple stimulus from the sleep environment influence nighttime rest? The results suggest that this possibility should at least be investigated further. However, one should not draw general recommendations from a small pilot study. With only twelve participants, the study is too small to reliably determine whether the effect also occurs in other age groups, in men, or in people with diagnosed sleep disorders.

No Proof Yet of an Effective Sleep Therapy, But an Interesting Approach for Sleep Research

This is the most important caveat. The study reveals interesting correlations, but it does not yet prove that hinoki oil can treat a sleep disorder. That would require larger randomized studies with significantly more participants. Furthermore, it would need to be investigated whether the observed effects persist over a longer period and whether they also occur in people with different types of sleep problems. The optimal concentration and method of application also remain unclear. The study used a very specific experimental setup involving three drops of oil and a defined distance from the head. It cannot automatically be concluded from this that more oil has a stronger effect or that other hinoki products produce the same effect.

Despite these limitations, the study is interesting from a chronobiological perspective. It shows that the sleep environment may be influenced by more than just light, noise, or temperature. Scents may also play a role—especially when they are repeatedly associated with the evening rest period. This aligns with a fundamental principle of sleep regulation: Throughout the day, the body processes information from its environment. Light is the most important cue for the circadian clock. However, other environmental stimuli can also be linked to specific times of day, behaviors, and states. A familiar, pleasant scent, for example, could become part of a recurring evening ritual. Whether this actually alters biological sleep regulation or primarily affects the subjective sense of relaxation remains to be clarified in more detail by research.

What Still Needs to be Clarified

The study has a second dimension. The roots of hinoki trees are often not fully utilized after timber harvesting. Extracting essential oils could therefore be a way to develop an additional product from this previously underutilized material. If the sleep-related findings are confirmed in larger studies, this could lead, for example, to the development of scented products for bedrooms or other relaxation areas. For the researchers, the study is therefore not only a sleep study but also an example of how forestry byproducts could be put to new use.

The results are interesting, but they are primarily a starting point for further research. Among other things, the following questions remain open:

• Which specific scents might be responsible for the observed effects.
• Whether the effect also occurs in men and other age groups.
• Whether people with chronic sleep problems react similarly.
• How long the oil needs to be used before changes occur.
• Whether the effects persist after use is discontinued.
• What concentration of the oil is appropriate and safe.
• Whether the effect is actually due to a change in sleep architecture or is partly influenced by relaxation and expectation.

It will therefore be particularly important to verify the results with larger groups of participants and more objective sleep measurements.

An Interesting Scent—But Not Yet a Sleep Aid

The Japanese pilot study provides several indications that hinoki root oil could influence sleep quality. Fewer nighttime awakenings, an altered pattern of delta activity during sleep onset, and improved subjective well-being in the morning paint an interesting overall picture. However, no recommendation for the treatment of sleep disorders can yet be derived from these findings. The study’s true significance lies, first and foremost, in the fact that it demonstrates just how strongly the immediate sleep environment can potentially affect nighttime rest. The scent of Japanese cypress could be another environmental stimulus whose effect on sleep and relaxation will be investigated more closely in the future. This is particularly exciting for chronobiology: Sleep does not occur in isolation, but rather in constant interaction with the environment.