Zen Meditation and Aromatherapy as a Core to Mental Health: Studies in Vietnamese Monasteries

Abstract

(1) Background: Meditation, originally stemming from the Buddhist tradition, has been the subject of extensive psychological research, with a growing body of literature exploring its effects on various mental and physical health aspects. (2) Methods: This article consists of two studies conducted in two Vietnamese Buddhist Monasteries. Study 1 investigated brain activity during Zen meditation among monks and nuns (N = 16) and a control group of non-practitioners (N = 3). Study 2, conducted on nuns (N = 13), explored the effects of meditation experience and essential oil inhalation on brainwave activity, with two measurements taken during short meditation sessions separated by inhalation. (3) Results: Study 1 showed that monks had higher alpha and theta activity during meditation compared to non-meditators. Moreover, advanced practitioners could transition from beta waves during mind-wandering to alpha waves within short 5-min sessions, indicating their ability to dynamically regulate brain activity even in brief meditative states. Study 2 found no significant differences between nuns with varying meditation experiences but revealed a significant decrease in theta wave amplitude after inhalation. (4) Conclusions: Together, these studies deepen our understanding of neural activity in advanced meditation.

1. Introduction

Epidemiological data from the Centers for Disease Control and Prevention (2022) indicate a rising trend in meditation practice among the general public: an especially significant increase in meditation use between 2002 and 2022 was demonstrated (Nahin et al. 2024). The development of this trend has been influenced by the mindfulness meditation clinical model (Kabat-Zinn 1982), which represents a secular approach to meditation. Such a framework has gained extensive empirical support for its effectiveness in therapeutic outcomes (Goldberg et al. 2023). Combined with the wide availability of study participants (contrary to research on traditional meditation practices conducted in monastic settings), this has contributed to its dominance in contemporary meditation research (Baminiwatta and Solangaarachchi 2021). However, this approach is not without limitations, as the secular interpretation of mindfulness encompasses a broad range of meditation techniques. Furthermore, most studies focus on short-term interventions (approximately one month), which are often without connection to the fundamental principles of Buddhist meditation (e.g., ethical foundations, wisdom traditions, and mental discipline). Thus, more research focused on the natural meditative environment is crucial for better understanding the long-term effects of meditation, its impact on neuronal functioning, and its connections with traditional ethical and philosophical contexts. This may contribute to enhancing secular meditation interventions and, in turn, improving mental health.

1.1. Zen Meditation

Among the various meditation techniques, Zen meditation stands out for its distinct approach rooted in the philosophical concept of nonduality and emptiness, which contrasts with other meditation styles such as mindfulness and Tibetan meditation based on visualizations and tantric practices (Różycka-Tran and Tran 2014, 2022). Zen meditation is defined “as the practice of mental concentration in which the reasoning process of the intellect is interrupted and consciousness is heightened by the exclusion of extraneous thoughts, except for thoughts that are the subject of meditation” (Różycka-Tran and Tran 2014, p. 125). While mindfulness uses a flexible focus on arousal and open monitoring, Zen meditation emphasizes an intense focus on the breath in the first stage and later detachment from one’s thoughts, sensations, and emotions. Its goal is to dissolve the distinction between observer and observed, achieving a psychological state in which consciousness and the object of meditation become inseparable (Cooper et al. 2022). In Zen meditation, practicing nonduality involves rejecting self-identification with any thought or sensation. Every feeling, idea, or emotion that refers to the Self is denied. These Zen meditation strategies vary in focus, cognitive approach, and motivational elements, particularly in moral orientation and analytical insight aimed at understanding the nature of reality. In Zen Buddhism, moral principles and the development of wisdom play a central role, while the secular mindfulness approach emphasizes the training of compassion and loving-kindness (Radoń 2017). This fundamental distinction in meditation philosophy likely underlies the differential effects on neural activity during these practices.

1.2. Techniques of Vietnamese Zen Meditation

One of the famous Buddhist traditions in Vietnam is the Truc Lam Zen school, which can be translated as “bamboo grove” in English. Truc Lam is a syncretic approach that combines Mahayana Buddhism, Chan (Chinese Zen), Theravada, and elements of local Vietnamese traditions (L. T. L. Nguyen 2021). As a result of Master Thich Thanh Tu’s efforts, Truc Lam has evolved into a more modern school of Zen Buddhism, emphasizing the integration of meditation into daily life, and making it more accessible to both monks and lay practitioners. Master Thich Thanh Tu aspired for Vietnamese Buddhism to embody the unique characteristics of Vietnamese culture, rather than being merely another imitation of Chinese Buddhism (L. T. Nguyen 2019). The main philosophy and history of this tradition have been extensively described in the books of Zen Master Thich Thanh Tu and some other articles (e.g., Różycka-Tran and Tran 2014, 2022).

Regarding meditation, there are four basic practice methods to guide practitioners to pacify the mind and emotions through breath counting and monitoring of thoughts, typically done while sitting in the lotus position. In the initial stage, practitioners count their breaths from one to ten, maintaining awareness of each inhalation and exhalation. This practice helps discipline the mind by keeping it focused on the breath (Thich Thanh Tu 2002). As the practitioner gains better control over their mind, they progress to the next strategy, which involves closely following the breath with full awareness (“When you breathe in, you know that you are breathing in; when you breathe out, you know that you are breathing out”; Thich Thanh Tu 2002, p. 33). In the more advanced stage, experienced practitioners observe the emergence of thoughts with each breath cycle. When a thought arises, they acknowledge its presence without engaging with it, simply observing the breath without reacting or becoming absorbed in the thought. This meditative process serves a dual function: it desensitizes individuals to their thoughts (through detached observation) and eventually removes those thoughts by continued focus on the competing response of breathing. Through this practice, individuals often feel relaxed, calm, and with a “mind emptied of internal chatter” (Shapiro and Zifferblatt 1976). Importantly, this practice is not limited to the sitting lotus position, it is applied in every moment of daily life activities such as cooking, cleaning, walking, and other activities (Różycka-Tran and Tran 2014).

1.3. Neurocognitive Bases of Meditation

Meditation, as a mental training practice, enhances an individual’s psychological abilities (Tang et al. 2015). However, different forms of meditation (such as: focusing attention on a specific object, thought, or breath, repeating words, visualization, or maintaining awareness of the surroundings) differ in their techniques, goals, and underlying neural mechanisms (Radoń 2023). Contemporary neuroscience research has examined the effects of meditation on the brain (Afonso et al. 2020). During meditation several structural and functional changes have been found in brain regions typically associated with attention control (the anterior cingulate cortex and the striatum), emotion regulation (including multiple prefrontal regions, limbic regions, and the striatum), and self-awareness (involving the insula, medial prefrontal cortex, posterior cingulate cortex, and precuneus; Tang et al. 2015; Tang and Leve 2016). Some studies have found that higher dispositional mindfulness is associated with greater grey matter volume in the right hippocampus, amygdala, and bilateral anterior cingulate cortex (ACC), but with reduced grey matter volume in the bilateral posterior cingulate cortex (PCC) and the left orbitofrontal cortex (Lu et al. 2014). These brain regions are responsible for attention, and self-referential processing, thus, meditation can significantly influence the improvement of cognitive functions, emotional regulation, and how an individual perceives and interpret themselves (Kong et al. 2016; Lu et al. 2014).

Moreover, some studies proposed that meditative states exhibit different brain dynamics than other relaxation states (Davis et al. 2023). For example, people who have some experience in meditation have lower gamma waves (>30 Hz) activity in the prefrontal areas and overall lower gamma power (Berkovich-Ohana et al. 2012). This result was interpreted as a marker of reduced default mode network (DMN) activity, which has been linked to lower mind-wandering (Aguerre et al. 2023). Other studies using EEG have shown that during meditation, there is a shift in brainwave activity, often characterized by an increase in alpha waves (8–13 Hz), typically associated with a relaxed and alert state of mind (Lehmann et al. 2001) as well as enhanced theta (4–8 Hz) activity, associated with deep relaxation and introspection (Aftanas and Golocheikine 2001). Also reduction in beta (13–30 Hz) activity was observed, particularly in the higher beta frequencies, often seen as a sign of decreased mental agitation (Tei et al. 2009). The role of alpha waves in meditation is to facilitate the transition into a more harmonious yet still conscious mental state (Katyal and Goldin 2021) and theta waves are characteristic of deeper meditation and a stronger detachment from external stimuli (Baijal and Srinivasan 2010), particularly among more experienced practitioners. In contrast, high beta wave activity may indicate states that are the opposite of those sought in meditation, such as excessive mental activity, stress, and tension (Thompson and Thompson 2003). Individuals with meditation experience have also demonstrated better connectivity between different brain regions, which was associated with improved cognitive and emotional processing (Aftanas and Golocheikine 2001; Jang et al. 2011).

However, the majority of studies on the neural basis of meditation’s beneficial effects have been conducted on secular populations (Tang et al. 2015). In response to this limitation, some researchers attempt to recreate sacred meditative conditions in laboratory settings (e.g., Manna et al. 2010). Such efforts cannot fully capture the unique nature of the meditative experience. Only a few studies have been conducted on monks within authentic monastic environments. For example, Japanese neuropsychiatrists conducted one of the earliest electroencephalographic studies on Zen meditation, demonstrating specific changes in the brain activity of monks, particularly an increase in alpha wave amplitude, which differentiates it from ordinary wakefulness (Kasamatsu and Hirai 1966). A study among Tibetan Buddhists in India indicated asymmetry in alpha and beta activity between the hemispheres and increased beta activity (Benson et al. 1990). In turn, among Tibetan monks in China, meditation has been associated with an increase in high-frequency brain wave activity (beta and gamma) in the prefrontal, parietal, and occipital regions, as well as with a decrease in delta wave activity in the prefrontal areas (Guo et al. 2022). In contrast, when researchers compared focused attention meditation (Shamatha, which involves concentrating on a single object) with analytical meditation (Vipassana, which involves reflecting on a specific topic, such as compassion or emptiness), the first one led to a greater increase in theta, alpha, and beta power compared to the baseline state and increase in alpha activity was most pronounced in the frontal and occipital regions (Neri et al. 2024). Vipassana meditation, on the other hand, induced less distinct changes in EEG activity. This highlights that different types of meditation influence brainwave dynamics in distinct ways. However, none of these studies have focused on Vietnamese monks and nuns from the Truc Lam tradition.

1.4. Meditation and Health

In the field of physical health, meditation offers several benefits, including improvements in the immune system and inflammatory processes through the reduction of cytokines, as well as promoting healthy aging through appropriate telomerase regulation, being beneficial in various multi-factorial diseases (Jamil et al. 2023). Some studies focus on the relationship between mental and physical health, demonstrating that conscious attention reduces amygdala activity, resulting in decreased anxiety, reduced depression symptoms, and improved self-esteem (Goldin and Gross 2010). Many studies provide a theoretical and empirical examination of the role of meditation in psychological well-being, self-regulation of behavior, and positive emotional states (Brown and Ryan 2003). Based on an examination of empirical studies across various methodologies, most authors conclude that meditation facilitates adaptive psychological functioning (Keng et al. 2011; Radoń 2023).

Studies have shown that regular meditation practice can lead to decreased cortisol levels (which is a stress hormone) and increased relaxation responses (Hofmann et al. 2010), as well as reduced insomnia symptoms (Kozasa et al. 2008). Meditation has also been found to improve cognitive control and the ability to sustain attention (Jha et al. 2007). It is linked with better emotional regulation, and enhanced empathy and compassion toward others, leading to improved interpersonal relationships and communication skills (Chambers et al. 2008). Regular meditation can reduce the perception of pain, which in turn leads to a better quality of life for individuals dealing with chronic pain conditions (Zeidan et al. 2010). Moreover, research suggests that meditation plays a crucial role in integrating interoception and exteroception, and can enhance sensory perception, including the sense of smell (Lefranc et al. 2020).

Because of strong empirical evidence, meditation has also been incorporated into therapeutic interventions for various psychological disorders (Goldberg et al. 2018). Such interventions and treatment methods include, for example, Mindfulness-Based Stress Reduction (MBSR), Mindfulness-Based Cognitive Therapy (MBCT), Dialectical Behavior Therapy (DBT) and Acceptance and Commitment Therapy (Teasdale et al. 2000). These treatment methods, grounded in meditation-related principles, have been shown to effectively reduce stress, anxiety, depression, alleviate symptoms of post-traumatic stress disorder (PTSD) and help individuals manage substance addiction or pain (Goyal et al. 2014; Keng et al. 2011; Saeed et al. 2019).

1.5. Aromatherapy

Not only can meditation be a useful practice for enhancing overall mental health, but it can also be complemented by aromatherapy. The aromatic compounds derived from natural products have been used for mental, spiritual, and physical healing since the dawn of time (Ali et al. 2015). In aromatherapy, these fragrant substances serve as a natural method for harmonizing the mind, body, and spirit (Sowndhararajan and Kim 2016). Incense burning is a significant element of cultural tradition and folk beliefs in Eastern countries.

In Vietnam, incense is widely used in both monastic and non-monastic contexts. A common practice among laypeople is maintaining a home altar for ancestors, where incense is burned as a gesture of spiritual connection (Jellema 2007). This tradition underscores the deep-rooted cultural and religious significance of scents, believed to bridge the material and spiritual worlds. In Buddhist monasteries, incense is highly valued for its purifying properties and plays a crucial role in meditation and rituals (Yadav et al. 2020). It helps create an atmosphere that fosters spiritual practice and deepens concentration. Moreover, aromatherapy is also integrated into traditional Vietnamese medicine, where various natural substances are used to support relaxation and enhance mood (Kraisintu 2003). Thus, aromatherapy can be seen as a modern extension of the traditional use of scents in Vietnamese spiritual and religious practices.

Myrrh is commonly associated with being a scented sacred plant, and in ancient times, it was used as a base for perfumes (e.g., kyphi) crafted for ritual purposes, especially in Egypt (Baser and Buchbauer 2020, p. 554). However, Vedic literature also contains passages suggesting the use of myrrh for ritual purposes and incense preparation (Penchala Prasad et al. 2008). Contemporary research highlights many beneficial properties of myrrh (Commiphora myrrh), including neuroprotective effects, anti-acetylcholinesterase activity, and sedative effects (Batiha et al. 2023). Similarly, Palo Santo (Bursera graveolens) may support meditation practices (Waller et al. 2012). Studies have also highlighted the relaxing properties of ylang-ylang (Cananga odorata) essential oil (Moss et al. 2008). Inhalation of this oil has been shown to reduce blood pressure and heart rate (Hongratanaworakit and Buchbauer 2004), as well as enhance alpha wave activity (Ishiguchi et al. 2008; Watanabe et al. 2013). Some studies suggest that ylang-ylang essential oil decreases information processing speed during tasks, may influence mood (Moss et al. 2008), and exhibit anxiolytic effects (Zhang et al. 2016). Patchouli (Pogostemon cablin) essential oil also exhibits stress-relieving and anxiety-reducing properties (Shin et al. 2020). Hence recent studies show its usefulness for prolonged sadness and mood disorders (Astuti et al. 2022). Studies also indicate that patchouli essential oil can reduce relative sympathetic activity, which is related to its calming properties (Haze et al. 2002). These effects are attributed to the essential oil’s main chemical components, as their fragrance compounds can cross the blood-brain barrier and interact with receptors in the central nervous system, making inhalation effective in treating various psychological and physical disorders (Sowndhararajan and Kim 2016).

1.6. Aim of the Study

This article aims to examine the psychophysiological effects of Zen meditation using electroencephalography (EEG), focusing on changes in brainwave activity during meditation. The study also investigates the effect of meditation experiences on brainwave patterns among nuns with shorter and longer meditation experiences and explores the impact of essential oil inhalation on alpha and theta wave activity.

As was mentioned above, different meditation practices influence brainwave dynamics in distinct ways. To the best of the authors’ knowledge, no studies have been conducted in this area among monks and nuns from the Vietnamese Truc Lam tradition. Moreover, there is a lack of research investigating whether scents used during meditation can assist meditators in achieving a state of relaxation more quickly. Thus, this study explores whether combining essential oils with meditation can enhance relaxation and facilitate an easier transition into a deeper meditative state, potentially offering a synergistic approach to improving mental well-being. Based on the available literature, the following hypotheses were formulated: alpha (H1a) and theta waves (H1b) will increase during the meditation; beta waves will decrease during the meditation (H2); the proportion of alpha to beta waves will increase during the meditation (H3). It was also hypothesized that alpha waves will increase after one minute of starting meditation (H4); nuns with shorter meditation experiences would exhibit higher levels of alpha compared to theta waves, in contrast to nuns with longer meditation practices (H5); and that the essential oil blend would lead to an increase in both theta (H6a) and alpha waves (H6b). To verify the proposed hypotheses, two studies were conducted.

2. Results

2.1. Study 1: Case Study Results from Monastery 1 and 2

The first monk was 53 years old and had eight years of meditation practice. The four 5-min trials of measurements were taken during the evening session (7:30–9:00 p.m.) on July 25 (Monk 1). Figure 1 shows the general percentage of waves and the proportion of relaxed or concentrated states of mind. The second measurement was taken from a 35-year-old monk with four years of meditation experience during an evening session (7:30–9:00 p.m.) on July 27 (Monk 2, Figure 2). A general increase in alpha waves and a decrease in beta waves during ongoing meditation can be observed (from 60% to 71% for Monk 1 and from 61% to 82% for Monk 2). The circular summary chart shows a percentage decrease in beta waves (from 10% to 4% for Monk 2) or stabilization (Monk 1).

To ensure gender balance in the study, EEG measurements were conducted on three nuns across three different sessions of the day. Nun 1, 38 years old, with 14 years of meditation practice, during the morning session on July 26, Nun 2, 53 years old, with 8 years of meditation practice, during the afternoon session on July 27, and Nun 3, 49 years old, with 11 years of meditation practice, during the evening session on July 28 (see Figure 3). As a reference point, EEG measurements were conducted on three beginners (laypeople staying in the monastery). Beginner 1 was a 26-year-old man with one year of meditation practice, measured during the morning session on July 24, Beginner 2, a 32-year-old with one year of meditation practice, measured during the evening session on August 18, and Beginner 3, a 75-year-old man with one year of meditation practice, measured during the morning session on August 19 (see Figure 3).

In turn, Figure 4 shows results from Truc Lam Thuong Chieu Monastery. The advanced monks were able to change beta brain waves after one minute of a wandering mind into a meditative state of alpha waves during short 5-min sessions. Based on the self-reported evaluations, all participants from this monastery indicated that they had achieved a high level of detachment from thoughts, sensations, and emotions.

The general results showed that the neural activity observed in monks and nuns during meditation (Figure 1, Figure 2, Figure 3 and Figure 4) was characterized by a predominance of alpha waves over theta and beta. It should be mentioned that the neuronal activity of the beginners, across all examined frequency ranges, remained at average levels.

2.2. Study 2: Aromatherapy Study in Monastery 1

To examine whether alpha waves increase after one minute of starting meditation during a five-minute meditation session, the Mann-Whitney U test was applied. Among nuns with short meditation experience, there were no statistically significant differences between the first and second minutes of measurement, Z = −0.52; p = 0.60. But among nuns with longer meditation experience, there were significant differences between the first and the second minutes of meditation, Z = −2.20; p = 0.028. For more details see Figure 5.

To examine whether nuns with longer meditation experiences differ in brainwave activity from those with shorter experiences, the Mann-Whitney U test was applied. The analysis did not reveal statistically significant differences between the two groups. However, although the results did not indicate a statistically significant difference, Z = −1.71; p = 0.086, the mean alpha wave amplitudes suggested that nuns with shorter meditation experiences exhibited higher alpha wave levels than those with longer experience (see

Table 1. The means and standard deviations for brainwave amplitudes in nuns with short meditation experiences and long meditation experiences.

Wave	Group 1			Group 2
	M	SD	Mean Rank	M	SD	Mean Rank
Delta	5.51	2.42	8.00	4.64	3.65	6.14
Theta	6.9	2.96	8.50	5.45	1.33	5.71
Alpha	10.21	5.92	9.00	5.82	2.81	5.29
SMR	6.42	4.97	8.33	2.76	1.63	5.86
Beta 1	4.82	3.45	7.67	3.28	2.28	6..43
High Beta	7.01	4.02	7.83	5.56	3.88	6.29

Note. Group 1 are nuns with shorter experience in meditation (N = 6). Group 2 are nuns with longer meditation experience (N = 7). M = mean; SD = standard deviation.

).

The Wilcoxon signed-rank test was used to determine whether brainwave recordings changed after inhalation of the essential oil blend. The analysis revealed a significant change in theta wave activity (Z = −2.48, p = 0.013). The amplitude of theta waves was significantly lower after the intervention compared to the pre-intervention state (outcome contrary to expectations). In 10 cases, the theta value after inhalation was lower than before, while in three cases, the theta value after inhalation was higher than before. There were no cases where these values were equal. For the remaining waves, no statistically significant differences were observed. Additionally, box plots were created to better illustrate the changes during meditation for alpha and theta waves during both measurements (see Figure 6).

3. Methods

3.1. Study 1

3.1.1. Participants

The study was conducted in two monasteries: Truc Lam Tay Thien in Vinh Phuc province in the North of Vietnam, primarily intended for beginners, and Truc Lam Thuong Chieu in Dong Nai province in the South of Vietnam, which is the destination monastery for advanced practitioners. The participants were monks and nuns permanently residing in Vietnamese Zen monasteries, aged between 26 and 75 years, with meditation experience ranging from 1 to 33 years. A part of the first monastery is open for short-term stays by laypeople seeking to deepen their meditation practice and find tranquility. Thus, the control group comprised Vietnamese laypeople (N = 3) who were staying at the monastery during the study period and expressed willingness to participate in the study. The researchers requested permission from the monastery leaders to conduct the scientific investigations. The monastery leaders approved the request and informed the monks and nuns about the study. All monks who expressed interest participated in the study. No incentives were offered to the participants. The first natural case study was conducted at Truc Lam Tay Thien Monastery, where brainwave measurements were taken from six monks, four nouns, and three laypeople. The second natural case study was conducted at Truc Lam Thuong Chieu monastery, where measures were taken from six monks studying in the Truc Lam Buddhist Academy, three experienced monks and one beginner. It is worth noting that Truc Lam Thuong Chieu is the final destination monastery for the venerable one-hundred-year-old living Zen master Thich Thanh Tu.

3.1.2. Procedure

The study was accepted by the Research Ethics Committee at the University of Gdańsk, opinion number: 42/2023/WNS. Research was conducted in July (Truc Lam Tay Thien—Monastery 1) and August (Truc Lam Thuong Chieu—Monastery 2). In a quiet meditation hall, monks and nuns sit on round cushions and practice meditation for 90 min three times a day: in the morning session (3:30–5:00 a.m.), in the afternoon (10:00–11:30 a.m.), and in the evening (7:30–9:00 p.m.). Figure 7 shows the monks wearing caps with recording electrodes while practicing Zen meditation in their natural environment. We asked participants if they experienced any discomfort; however, the electrodes did not interfere with Zen meditation, allowing for stable and long-lasting recordings.

The measurement was carried out using a BrainBit Flex mobile cap to monitor brain activity, with a certificate of conformity (No. ET-22050215EC). BrainBit Flex is a four-channel consumer EEG device with Bluetooth LE connectivity with four dry, gold-plated electrodes. The reference and common electrodes are designed as ear clips. In our study four electrodes were applied on the scalp of the frontal (Fp1, Fp2) and occipital (T3, T4) regions of the head (according to the International EEG Electrode System 10–20). We monitor EEG brain waves in real-time using the mobile application α EEG Waves (version 1.0.1. based on BRAINWAVE software by BrainBit Inc. and Brainwave360). Signal transmission is conducted via Bluetooth, as the monastery, located deep in the jungle, lacks an Internet connection.

The procedure in Monastery 1 EEG measurements were taken in real-time during meditation sessions. The summary of the recording, covering four trials of 5 min of meditation, was presented in pie charts (see e.g., Figure 1 and Figure 2). Since all participants in the first monastery were measured during full 90-min sessions, in the second (more advanced) monastery, we chose to assess the dynamics of meditation in shorter 5-min sessions to examine potential differences between long (90-min) and short (5-min) meditation sessions. Thus, the procedure in Monastery 2 consisted of two stages: a one-minute calibration (to stimulate the “monkey mind”), during which participants were instructed to engage in mind-wandering. A light tap on the shoulder with a pen signaled the transition to the second stage, which was a five-minute core meditation session. During this stage, participants meditated in their usual manner. Then, participants provided a subjective evaluation of their meditative state on a 3-point scale, where 1 indicated low and 3 indicated high detachment from thoughts, sensations, and emotions.

The graphs display changes in wave activity over time (see Figure 1, Figure 2, Figure 3 and Figure 4). For each of the rhythms (alpha, beta, and theta), a percentage is shown relative to a value of 100%. Depending on the change in the percentage ratio, the fill color of the zone changes accordingly with colors associated with specific rhythms: grey (theta rhythm) for deep relaxation, blue (alpha rhythm) for relaxation, and orange (beta rhythm) for concentration (a wandering mind). We specifically investigated changes in alpha, beta, and theta waves. We did not analyze delta and high-gamma frequencies due to their susceptibility to artifacts (Berkovich-Ohana et al. 2012).

3.2. Study 2

3.2.1. Participants

During the study, approximately 30 nuns were residing in the Truc Lam Thay Thien monastery. All of them were invited to participate and 13 of them (participation rate 43%) agreed to take part in the research. The youngest participant was 27 years old, and the oldest was 74, with an average age of M = 46.38 (SD = 15.52). Regarding the duration of meditation practice, six nuns had been practicing for less than 7 years: two for 1 year (15.4%), one for two years (7.7%), two for 5 years (15.4%), and one for 6.5 years (7.7%). The remaining nuns reported longer periods of practice: one for 9 years (7.7%), one for 10 years (7.7%), two for 12 years (15.4%), and two for 15 years (15.4%). The longest reported duration of meditation practice was 24 years, (7.7%). None of the participants reported any neurological issues or olfactory impairments. No incentives were offered to the participants. Those who expressed interest in their results received a brief verbal summary along with an interpretation.

3.2.2. Procedure

Data were collected in November 2024. The EEG system utilized in this study included the EEG Sensor and the PROCOMP2™ Encoder with BioGraph Infinity Software (T7400M) developed by Thought Technology Ltd. (Montreal, QC, Canada). The equipment is equipped with an EEG-Z sensor, which features an input signal range of 0–200 µV and a bandwidth of 2 Hz to 1 kHz. This dual-channel EEG encoder is capable of recording data from two cup electrodes (made from AgCl) and two ear electrodes.

EEG recordings were collected from the nuns staying at the monastery. The recordings were conducted using two head electrodes placed at the Fz and Cz positions and two ear electrodes. The electrodes were attached using conductive adhesive paste. The procedure involved measuring brain waves for 5 min during meditation, followed by a 10-min no-meditation period during which participants smelled a blend of essential oils (1 drop applied to a water-dampened cotton pad). The essential oil blend consisted of myrrh (Commiphora myrrh), Palo Santo (Bursera graveolens), patchouli (Pogostemon cablin leaves), and ylang-ylang (Cananga odorata flowers) oils. After the inhalation, the nuns were asked to resume meditation, during which another 5-min measurement was conducted. None of the study participants reported any contraindications to participation (e.g., loss of smell, aversion to certain scents, etc.). All recordings were conducted in the meditation hall in the early afternoon. Nuns meditated in the “lotus position” (a traditional sitting meditation posture, with crossed legs and feet resting on the thighs) with closed eyes. Participants were instructed to meditate as they normally do. Considering that scent preferences can influence psychophysiological responses (e.g., Sowndhararajan and Kim 2016), the participants were asked before inhalation whether the scent was acceptable to them. None of the nuns reported scent aversion. After completing the measurements, the recordings were cleaned of artifacts.

3.2.3. Statistical Analyses

Means, standard deviations, percentages, and non-parametric tests were calculated using IBM SPSS 29.0. All tests were two-tailed, and the significance level was set to α = 0.05.

4. Discussion

4.1. Study 1

Study 1 focuses on changes in brainwave activity during meditation. After analyzing and comparing the data from two monasteries from different groups of participants, it was revealed that a series of EEG changes occurred during attentive awareness during a Zen sitting session. Generally, during the first 5–10 min of meditation, well-organized alpha waves became dominant, primarily in the frontal regions. Initially, these alpha waves alternated with short bursts of an activating pattern. However, as meditation progressed, a relatively stable period of persistent alpha waves ensued, with the appearance of theta waves becoming distinct during these stable periods (H1a and H1b sustained). This phenomenon was in contrast to the control subjects, who exhibited a long-lasting activating pattern of beta-dominant EEG with short bursts of small alpha waves.

Advanced monks were able to shift from beta brain waves, associated with mind-wandering, to a meditative state characterized by alpha waves within just one minute during brief 5-min sessions (H2 sustained). It suggests that Zen practitioners can influence the brain waves not only during long meditation but also increase the dynamic of meditation in very short trials. For more details, see (Kurek et al., forthcoming).

The results indicated also that neural activity in monks and nuns during meditation was predominantly characterized by alpha waves rather than beta waves, particularly when compared to beginners (H3 sustained). Long-term meditation practice may lead to neuroplastic changes in the brain that promote the dominance of alpha waves during meditative states, which aligns with previous research on the long-term effects of meditation (Radoń 2017).

4.2. Study 2

Study 2 aimed to explore the effects of the meditation experience and the inhalation of an essential oil blend on brainwave activity during meditation. The results confirm that alpha waves increase after one minute of meditation. However, this effect was statistically significant only among nuns with long-term meditation experience (H4 partially confirmed). The increase in alpha waves after just one minute suggests that meditation can have an almost immediate impact on brain activity, particularly in experienced practitioners. This is consistent with previous research findings, which have demonstrated that experienced meditators were able to transition from a resting state to meditation within one minute (Nair et al. 2017). It confirms that long-term engagement of attention-related brain networks through repeated practice can result in lasting modifications in functional connectivity within the associated neural circuits (Hasenkamp and Barsalou 2012).

While no statistically significant differences were observed between nuns with shorter and longer meditation experiences (H5 not confirmed), trends in the data suggest potential differences in alpha wave activity. Alpha waves reflect a state of relaxation and a transitional phase between wakefulness and meditation. Among beginners, higher alpha amplitudes may indicate that they are in a more relaxed state rather than in deeper meditation, which is typically characteristic of more experienced practitioners (e.g., Takahashi et al. 2005). However, with such a small sample size, it is challenging to obtain statistically significant results, thus it requires further research on a larger group of nuns or monks.

Contrary to what was assumed, the analysis of brainwave activity before and after the inhalation of the essential oil blend revealed a significant reduction in theta wave amplitude (H6a not confirmed). There were no statistically significant changes in alpha wave activity (H6b not confirmed). Previous studies have indicated the relaxing effects of the essential oils used in this study, including their ability to increase alpha waves and lower blood pressure (e.g., Hongratanaworakit and Buchbauer 2004; Watanabe et al. 2013). However, in the case of the nuns, inhalation did not lead to an increase in alpha or theta waves. This may be due to the fact that the nuns do not always meditate accompanied by the scent of incense and when they do, the incense is typically placed on the altar at a considerable distance from where they are seated. Thus, despite their approval of the fragrance, it may have disrupted their ability to maintain a deeper meditative state.

4.3. General Discussion

Given results align with findings from other studies that have utilized advanced neuroimaging techniques (see e.g., Tang et al. 2015) and with the four stages of Zen meditation described by Kasamatsu and Hirai (1966): the appearance of alpha waves (stage I), an increase in alpha amplitude (stage II), a decrease in alpha frequency (stage III), and the emergence of rhythmic theta patterns (stage IV).

What remains intriguing is the distinct configuration of frequency ranges observed across each study group. A comparison of these configurations suggests that the nuns had the relatively best results (highest alpha wave intensity and average beta, but weakest theta). On the other hand, the frequency range configuration of monks indicates high meditative advancement (relatively lowest beta range intensity and highest theta, but slightly lowest alpha). It is essential to recognize that the effectiveness of meditation can vary from person to person (Radoń 2023). Therefore, not all studies produce consistent results and it is crucial to analyze individual differences among participants.

Moreover, the results indicate that the most effective strategy for measuring the dynamics of neuronal changes during meditation involves short-term meditation, one minute of mind wandering followed by four minutes of focused meditation. Using such a timeframe is beneficial because practitioners can recall when their meditation was subjectively better or weaker. Additionally, during short meditation sessions, symptoms of fatigue are less likely to emerge, as prolonged meditation can be detrimental to particular individuals (Radoń 2023).

Furthermore, an increase in alpha waves after just one minute suggests that the brain quickly transitions into a state of relaxation and focus. Many modern stress reduction programs and mindfulness-based therapies, such as MBSR and MBCT, incorporate short meditation sessions (Goldberg et al. 2018). Since alpha waves increase within a minute, this indicates that even brief meditation practices can be effective, highlighting their potential value in clinical applications. Therefore, this may serve as a practical guideline for beginners, showing that even short sessions can influence brain activity.

Regardless of the preferred technique, meditation has a profoundly positive impact on mental health, enhancing stress resilience, improving emotional regulation, strengthening concentration, and supporting overall well-being and psychological balance (Cahn and Polich 2006; Keng et al. 2011). Nonetheless, meditation is not the only practice with such benefits, as aromatherapy may serve as an effective complementary method supporting other techniques aimed at enhancing well-being (Freeman et al. 2019; Ramsey et al. 2020).

Despite the vast cultural differences between traditional (Buddhist) and modern (psychological) approaches to achieving enlightenment in both Eastern and Western cultures, modern technology seems to be a bridge. While the traditional path toward attaining the state of Bodhi relies on a natural and lengthy process of meditation directed by instructions (Buddha, Dharma) practiced in a monastery (Sangha), our studies confirm that modern technology in the field of psychology can serve as valuable support for investigating meditation-related neurophysiological processes.

4.4. Strengths, Limitations, and Future Studies

Regarding the strengths of the study, to the best of the authors’ knowledge, this is the first study with brain activity measurement conducted on monks and nuns from the Vietnamese Truc Lam tradition. Moreover, previous research has also not examined the relationship between scents and brain activity during meditation among nuns, making this study a significant contribution to understanding these effects. Previous research conducted outside meditative and monastic contexts suggests that certain essential oils can promote relaxation or enhance concentration, leading to changes in brain wave activity (Sowndhararajan and Kim 2016). Therefore, exploring the synergy between meditation and aromatherapy presents an innovative approach, allowing for an investigation of the potential combined effects of these two practices in supporting mental health. However, it remains unclear how these effects manifest across different religious groups, as Vietnamese nuns did not exhibit an improvement in achieving a deeper meditative state.

Our research, conducted using mixed methods, combines case study analyses in the natural environment of monks and nuns with statistical methods. However, accessing such a research population is challenging thus the study was conducted on small groups of monks and nuns, which limits the generalizability of the findings. A larger sample size would increase statistical power and provide more robust results. Although participants were asked about their approval of the scent, individual differences in scent preferences and sensitivity could have influenced their responses and brainwave activity. Future studies should consider allowing time for participants to acclimate to the scent, for example, by applying essential oils during standard meditation sessions for some time, e.g., a week. Additionally, the effects of such an essential oil blend should be tested on laypersons with varying levels of meditation experience.

Moreover, in the first study conducted in Monastery 2, participants were asked to provide a subjective evaluation of their meditation. The researchers aimed for the simplest and shortest possible form of participant feedback. However, such subjective assessments are not a precise indicator of meditation (in this case, referring to the evaluation of how detached participants felt from their thoughts, sensations, and emotions), as they are susceptible to individual biases. In future studies, when using such a subjective evaluation, it would be advisable to compare the results with raw data on brainwave activity.

Individual differences in brainwave patterns may also contribute to the observed results. Additionally, the presence of the researcher and the equipment during meditation could influence participants’ level of relaxation, focus, or ability to fully engage in the meditation process.

Concerning the measurement times, in the first study, recordings were scheduled in alignment with the monks’ and nuns’ regular meditation routines. In the second study, all recordings took place in the early afternoon—a time when the nuns were not yet fatigued by their daily responsibilities. However, to the best of the authors’ knowledge, no studies have specifically examined how the time of day during which meditation occurs influences brainwave activity. Therefore, this issue should be addressed in future research, particularly among experienced meditators.

Another limitation of the study are EEG tools used for these studies. The first study allowed for quick and accessible monitoring of brain waves but lacked full raw EEG data. The second study provided a more precise measurement of brain activity, but it was limited by the number of electrodes. Future research could benefit from more advanced EEG systems, enabling both comprehensive raw data recording and a more detailed spatial analysis of brain activity.

Looking synthetically at the results in the context of other studies that utilized highly sophisticated neuroimaging tools (Radoń 2022, 2023), it should be noted that analyzing the activation of different frequency ranges of neuronal activity alone does not provide accurate information. This issue is because there are no established standards for measuring the extent of activation in individual frequency ranges. Furthermore, solely considering the overall magnitude of neuronal activation, without considering the activation in different brain areas or individual sensors, offers a simplistic representation of a person’s mental state (Radoń 2023). This issue arises not only from the limitations of the EEG used (even a professional one, which may consist, for example, of 120 sensors) as it primarily measures neuronal activation near the surface of the skull (estimating activation for approximately 1000 neurons per sensor, with deep brain structures remaining inaccessible). However, despite these limitations, EEG remains a powerful tool, even in its simplified form, for studying brain activity in meditation, especially when used in naturalistic settings such as monasteries. EEG tools that provide clear graphical feedback to users can serve as a valuable resource for improving meditation skills, particularly for individuals who do not have the opportunity to train under the guidance of meditation masters in an original and peaceful monastery environment.