The tangerine essential oil caused statistically significant effects to the brain in different brain regions. It increased brain-wave sub band of beta wave power but decreased the alpha wave power [
10]. This study shows the vigilance effect of undiluted tangerine oil. Beta wave power is observed when awake, especially active thinking, focused, high alert for mid beta wave power and fast idle and musing for low beta wave power [
14]. Alpha wave power is observed during several states of mind or activities. Slow alpha wave power correlated with relaxed state, eyes close condition, calm state, and resting state. Fast alpha wave power correlated with idling state. The reduction of this brain wave power suggests a brain functional shift to a more alert state of mind of beta [
6]. Limonene is a major component in tangerine oil detected by GC-MS. Another study found that autonomic nervous system was increased by (+)-limonene, which was observed by the increase of sympathetic activity parameters [
13]. Sowndhararajan et al. also reported beta wave power increase by (+)-limonene inhalation [
15]. The increase of alertness by tangerine oil or limonene might be because of several underlying brain processes and the psychological basis. Brainstem–thalamo–cortical pathways, suprachiasmatic nucleus—circadian rhythm, hypothalamo–pituitary–adrenal axis and limbic system, as well as the metabolic system and substrates need to be further investigated to explain the underlying mechanisms of these volatile fragrances [
16]. The vigilance effect of tangerine oil is similar to rosemary oil. Previous study suggested that volatile organic in
Chrysanthemum indicum, 1,8-cineole, can increase the low beta at several sites, excluding frontal, parietal and occipital. By contrast, only frontal is detected in the present study [
17]. In the case of mid beta wave of 15–20 Hz with high significance in active thinking and high alertness, Kim M. has shown that mid beta wave was increased during inhalation of (+)-α-pinene and terpinolene, which is correlated to decreased mid beta, indicating relaxation and reducing stress [
15,
18]. In addition, the effect of essential oil of
Inula helenium root decreases the mid beta but only in the parietal region [
19]. Each region of the brain is responsible for different functions from homeostatic functions to higher functions. In this study, we divided electrodes into four groups representing frontal, centro-temporal, parietal, and occipital regions. The frontal area is responsible for many important cognitive functions such as working memory, calculation, motivation, and executive function. This brain region collaborates with other brain areas in other deep nuclei to do these elaborative functions. The alteration of brain activities in this brain area may correlate with the mentioned duties. The temporal lobe and central area of the brain are responsible for limbic system-related function, motor function and sensory function. The parietal region is responsible for associative function, and the occipital lobe is responsible for vision. Previous studies showed increased slow alpha in different regions. Neroli and grapefruit oils influenced activity in the occipital region, which indicates relaxation state and helps to reduce cortical deactivation [
20]. The mixing of bergamot and lavender leads to the increase of relative fast and slow alpha waves in the prefrontal region, which is also helping to relax [
21]. Orchid oil increased the parietal region activity [
22]. Low beta is decreased during inhalation of (+)-α-pinene, which is specific in male subjects. The increase in beta wave has also been correlated with an enhanced drowsy state [
17]. The decrease of fast alpha wave power was observed during resting and concentration period of mind. The other studies also showed the different results. When the subjects were exposure to agarwood incense, the fast alpha wave was significantly increased and increased during inhalation of (+)-α-pinene and (+)-β-pinene [
17,
20]. Moreover, the present study showed an emotional state after inhalation of the undiluted oil, which happens in the same way as brain wave power. The visual analog score of orchid oil inhalation also showed higher scores in elegance and freshness [
18].
In this study, we did not investigate the related functions of each brain area; however, we investigate a function related with arousal level. The sleep onset-latency was measured in this study to further extend the knowledge related with sedative function of tangerine oil. We prepared the oil in the level of subthreshold and threshold concentration instead of suprathreshold, undiluted oil. We found that both the subthreshold and threshold showed an interesting function opposite to undiluted oil. It showed a sedative effect in these ranges of dilutions. We hypothesized that the oil may help sleep function. Only female subjects were recruited in this study and the result confirms the hypothesis. The result briefly showed the effect on the alertness system. Sleep onset latency decreased in female subjects.
Lavandula angustifolia essential oil also decreased sleep onset latency, which is like the threshold level of tangerine oil [
23]. In vivo study revealed that limonene significantly increased γ-aminobutyric acid (GABA) and other neurotransmitter changes [
24]. GABA is acting though GABA
A receptor, which is potentiating chloride current resembling alcohol, barbiturates, and benzodiazepines. Moreover, other neurotransmitters were reported to change after limonene administration such as dopamine (DA), serotonin (5-HT), and glutamate (Glu). These neurotransmitters play an important role related with the waking system of the brain so the direct effect of the oil to neurotransmitter function may be the underlying mechanism related with its sedative and vigilance effects. This detailed mechanism is still needed to be investigated. The gender difference was indirectly observed in this study. Previous studies also showed different brain waves between female and male during inhalation of (+)-α-pinene and (+)-β-pinene as well as black pepper essential oil [
17]. Moreover, the other studies revealed that reproductive hormones and odor reception are the cause of different responses between genders [
25]. Different olfactory sensitivity between genders of many animal species may be caused by the physical receptor [
26]. In other studies, corpus collosum of females is larger than males; in addition, the development of cerebral lateralization of females is also faster than males [
27,
28]. Furthermore, the other study showed that females responded to the molecules of the essential oil more than male did, and Haehner et al. also revealed that responses of females and males to grapefruit or a combination of orange, lime and lemon fragrances are different [
29,
30]. In addition, sex-related performance can be explained by reproductive hormone influences on odor perception. Doty et al. explained that alterations of olfactory performance were observed in different states of hormone reproduction, such as gonadectomy, hormone replacement therapy, pregnancy, and menstrual cycle [
31]. All the above are consistent with the present study because females have more brain activity during inhalation of tangerine essential oil compared with male subjects. Cognitive tasks should be further examined in the future to study the related brain functions. A previous study showed that San-Jo-In essential oil can increase attention and relaxation with a significant increase in fast alpha power at the left prefrontal, right prefrontal and left frontal regions in the same way as the effect of lavender and bergamot [
32]. Moreover, alpha increase in the frontal brain region was related to moods, helping to decrease tension and stress states [
6]. With regards to the effect of increased beta wave power in frontal regions, other studies showed that it was related to alertness and concentration state including memory task performance improvement [
33,
34]. Kim et al. showed that alpha wave was increased during inhalation of (+)-α-pinene and (+)-β-pinene, which may improve brain functions and enhance relaxation. This study only measured overall brain responses and related functions such as sleep function. The mechanism related with brain communication is still obscured during essential oil inhalations.