In a 2019 systematic review and meta-analysis, Pi-Chu Lin and colleagues from Taipei Medical University in Taiwan evaluate the effects of aromatherapy on sleep quality (Lin, et al., 2019). In their selection of studies, they limit the search to RCTs (Randomized Controlled Trial) and clinical studies conducted with human subjects aged 20 and above, using keywords “aromatherapy”, “sleep quality” and “insomnia” and limited to inhalation, massage, bath and liquid oral intake. The studies should also have found an intervention effect and do so using sleep scales, such as the Pittsburgh Sleep Quality Index (PSQI). Such scales invite the participants to record self-reported sleep parameters.
The authors fetched 337 articles, of which 31 studies meet the set criteria, accounting for 2389 tested subjects. That is a major step forward compared to a 2015 systematic review and meta-analysis by Hwang and Shin (Hwang & Shin, 2015). In this 2015 study, the Korean authors identified 245 studies from which they withheld 12, with publication date up to 2012, mainly conducted in Korea. The lack of international studies in the selection procedure seems to be a shortcoming of the 2015 review.
The species tested range from Rose (Damask) (Rosa × damascena), (Sweet) Marjoram (Origanum majorana) and Chamomile (Roman) (Chamaemelum nobile or Anthemis nobilis) to the Citrus genus. Lavender and Bergamot appear to be the most tested extracts. 17 studies (56%) use a single extract and 14 studies (43%) use a mixture of several extracts. 15 studies (48%) evaluate inhalation, 8 studies (25%) massage, bath and oral intake and the remaining 8 studies (25%) various combinations of application methods.
The researchers also perform various subgroup analyses, including the use of a single extract versus oils combined in mixtures, the method of application and the methodological quality. The authors note a high level of heterogeneity in the studies investigated.
Pi-Chu Lin and colleagues conclude that aromatherapy can improve sleep quality, confirming the 2015 results of Hwang and Shin.
Pi-Chu Lin and colleagues conclude that aromatherapy can improve sleep quality, confirming the 2015 results of Hwang and Shin (see above). The combination of application methods delivers better results than mere inhalation or no inhalation (massage, bath and oral intake). At the same time, the authors note that the question of whether or not a synergistic effect occurs when combining an olfactory effect (thru inhalation) with a cutaneous effect (thru massage) requires further investigation. Synergy means that the combined effect is greater than what the individual elements (in this case: application methods) deliver. The concept is commonly used in aromatherapy to describe the combined action of either multiple components or multiple extracts in a mixture .
The use of combined oils in a blend also yields better results than the use of a single extract. Since blending is an important concept in aromatherapy, I highlight a study that compares the effect of a mixture of extracts with the use of a single extract.
A 2017 Taiwanese study by Kao et al. investigates the effect of True lavender EO (Lavandula angustifolia), Clary sage EO (Salvia sclarea) and (Sweet) marjoram EO (Origanum majorana) on sleep quality in 132 career women (Kao & et. Al., 2017).
Linalyl acetate, the ester of the monoterpene alcohol linalool and acetic acid, appears to play an important role in the author’s selection of essential oils. The biochemical composition of the extracts, used in the study, is not mentioned. Linalool and linalyl acetate are constituents, to a varying degree, of the extracts: True lavender EO: linalool (31.73%), linalyl acetate (33.29%) (# OF20610); Clary sage EO: linalool (17.13%), linalyl acetate (65.42%) (# OF24207); (Sweet) marjoram EO: linalool 1.7-3.3%, linalyl acetate 7.4-10.5% (Tisserand & Young, 2013, p. 346). In a single sentence the authors state that Clary sage EO contains sclareol, that sclareol is chemically similar to estrogen and thus enables balancing the female endocrine system. In the literature cited following this statement I did not find any information to support this statement. Read also: “The professional’s bookshelf“.
A first intervention group uses True lavender EO (group 1); for a second group, a 1:1:1 blend of the three extracts is prepared (group 2); distilled water is used for the control group. The intervention consists of diffusing in the participant’s own bedroom during 20 minutes, three times a week for 4 consecutive weeks. A third intervention group receives acupressure massage focused on 17 acupuncture points for 45 minutes, once a week for four weeks (group 3). The effects of the interventions were measured with the Pittsburgh Sleep Quality Index (PSQI) and the Short Form (36) Health Survey.
All intervention groups note improved sleep quality and quality of life (QoL). The improvement in the quality of life is greater in the blend group (group 2), compared to the Lavender group (group 1). The improvement in sleep quality and in quality of life is greater in the blend group (group 2) and in the acupressure massage group (group 3), compared to the Lavender group (group 1). In comparison with the Lavender group (group 1), acupressure massage (group 3) only notes an improvement in sleep quality.
 Jennifer Peace Rhind published a 2016 book on the synergistic effects of blending: Rhind, P. J. (2016). Aromatherapeutic blending – Essential oils in synergy. London: Singing Dragon. Read also Tisserand & Young, 2014, pp. 24-25.