Aromatic herbs for stress and mood

We know there are many herbs that can affect mood. Some, like coffee, are strong energizers. Others, like kava, are powerful relaxers. But there is a whole class of medicinal plants--the aromatic nervines--that act as tonics to the nervous system and spirit, helping to find the place of balance between enlivening inspiration and gentle relaxation. These nervines include lemon balm, linden, lavender, rose, skullcap and more, and the way they work in the body is fascinating: at the tissue level, they both support healthy tone and soothe excess tension. And more generally, their smell and their effects in the body synergize to deliver reliable benefits: they uplift the spirit and support a mellow mood. By and large they don't contain alkaloids--the molecules we usually think of as psychoactive drugs. What are these sweet-scented herbs up to? 

When you find aromatic plants in the field or garden, you can draw out their smell by gently rubbing leaves or flowers. As your fingers bruise the plant, you're breaking trichomes: microscopic surface structures loaded with volatile molecules that release their contents into the air when they're damaged. Among these molecules, we find terpenes and phenolics, all blended in different combinations depending not only on the species of plant but also on the time of day, the season, and the genetic ancestry of the specific individual. The blend is known as the essential oil of the plant. These trichomes are found on an incredible variety of herbs, from mints, to parsley family plants, to Cannabis. All contain some of these volatiles, adding up to significant percentage of the dry weight of a plant: certain mints (like peppermint) contain between 1% and 3% menthol ¹, while caraway (a cousin of dill and parsley) can contain up to 7% carvone ².

Why do aromatic plants make these volatile molecules to begin with? One of our first clues comes by observing how they're released: it is after damage to the trichomes that the scent appears in the air. Researchers like Richard Karban at UC Davis have observed how neighboring plants (Artemisia tridentata, specifically) are attuned to these scents, and increase the production of defensive compounds when they detect them nearby. What is even more fascinating is that this response is strongest if the scent--with its precise cocktail of volatile molecules--comes from a genetically-related plant (not necessarily the closest)³. And, as Ted Farmer and his team at the University of Lausanne in Switzerland discovered, these cocktails work by connecting with receptors on the surface of leaves. This triggers a wave (technically a depolarization wave in neuron-like structures) that travels deep into the plant to activate those defenses ⁴. Herbs use aromatic molecules to communicate. They recognize kin. They warn each other, and spring into action.

It is remarkable to note that, inside plant leaves, this warning is carried on an electrical wave. The structures and mechanisms involved are very similar to those in our own neurons, so you might be tempted to think that we, too, can use these aromatics to send signals across our nervous systems. But while some of our neurotransmitters share structural features with phenolic aromatics from plants, there is a crucial difference: plant volatiles lack the atom nitrogen as part of their chemical structure. So it would seem that, in most cases, these plant signals don't work in quite the same way in human beings--they aren't direct triggers for our nerve signal pathways.

But there is one place where we do find a direct connection: our sense of smell. In the mucous membrane of the nose are thousands of receptor sites, each fine-tuned to a different type of volatile molecule. When triggered, these receptors activate neurons that send signals to the olfactory cortex of our brain: we perceive a smell, unique and based on the cocktail of aromatics in the air. Almost immediately our limbic system, that central site of connection between sensation, organ function, and memory, layers meaning into the experience by linking it with past associations and building in emotional content. Think of key smells from your childhood, and how, when  you encounter them, your past experience comes flooding back. This is the power of the direct link between aromatics and our nervous system.

Engaging our brains through smell seems to set the stage for gentle relaxation and focused attention. This can be very useful! We can access this through a range of techniques, starting with incense, potpourri, and mulling spices (and their more modern cousin, the aromatherapy diffuser). Many strong-smelling plants are traditionally used this way. The volatile molecules in pine (a mixture of terpenes, rich in pinenes) contribute to its uplifting aroma ⁵. Lavender volatiles (terpenes again, with the majority being linalool) seem to be responsible for the plant’s calming scent ⁶. Lemon balm's volatile chemistry (featuring the terpene citronellal, and linalool again) is similar ⁷. Even bergamot oil (from Citrus bergamia, source of the characteristic scent of Earl Gray tea) is sought after for these aromatic qualities ⁸. There are many other examples, but the general principle seems to be the same: terpenes and simple phenolics from plants speak to our spirits, even when all we do is smell them.

Another great way to deliver some of these volatile molecules to our noses is by steeping aromatic herbs in hot water: a simple cup of tea. You can detect them after a few seconds of steeping: the heat of the water liberates them from their trichomes and they rush upward on the hot steam (one reason you want to put a lid on that steeping cup). But drinking the tea takes the power of aromatic plants a step further, allowing their chemistry to come into contact with the mucous membranes and muscles of the GI tract, and later, other muscles in the body too. This is where things get really interesting, and we see that, even without a direct connection to our nerves, volatile molecules may have an effect on our mood and perception of stress.

When volatiles come into contact with muscle tissue, you get two effects. At first there is a mild pungency that "wakes up" the tissue. This effect varies from plant to plant, and results in a small, temporary increase in contractility. The muscles tighten slightly. This is followed quickly by a more pronounced, and prolonged, antispasmodic action that brings the muscle to a normal, relaxed state of tension and tone (resulting in the more familiar effect of aromatic plants). Maria Lis-Balchin, affiliated with London's South Bank University, has investigated this and characterizes the overall effect as a "reset" of muscle tone that helps our smooth muscle to find a healthy balance point of tension ⁹. Others have gone into more detail, describing the mild calcium-channel blockade in muscle cells that comes from contact with the botanical terpenes of peppermint oil in gut tissue ¹⁰ or terpene-rich motherwort extracts in cardiovascular tissue ¹¹. This may be part of our aromatic herbs' mechanism of action, but in the end, what matters is that a warm cup of aromatic tea first engages your senses, and then supports healthy tone of the smooth muscle in our GI tract and other muscles in our body.

To maximize these benefits of aromatic plants, it helps to pay attention to two details: first, recently-dried herbal material often packs more of a punch when brewed than does fresh material. Try crushing and steeping some fresh lemon balm leaves and compare the tea to one made from dry leaves: a given weight of dry herbs has a lot more terpenes than a similar weight of fresh herbs. The tea tastes stronger and more aromatic. But second, the type of liquid the herbs steep in also determines how many volatiles are extracted: essential oils are not very water-soluble, and steeping the herbs in strong alcohol can also extract more aromatic chemistry. This is partly why herbalists will take these strong-smelling, nervine herbs and make tinctures: alcohol is a more versatile solvent, and keeps those terpenes and phenolics in solution better than water alone. With a tincture, you can add small amounts--1/4 to 1 full teaspoon--to seltzer or warm water to liberate the volatiles and enjoy the benefits.

It might seem obvious that, by setting the stage for a relaxed gut and cardiovascular system, we will help support a normal mood and reduce the effects of stress. But the reason why is not immediately obvious. To understand, we have to remember that the limbic system is constantly receiving information from all our organs, including the heart and gut. This information includes the degree of tension and tone: during stress, when circulation to the hands and feet decreases and the gut clamps down, the limbic system detects this high degree of internal tension. Conversely, when we're relaxed, a whole different set of signals is sent up to the brain. The degree of internal tension and the information from our senses come together with our memories to help us decide: are we stressed, or not?

This blending of internal state and external input has been termed the allostatic load by Bruce McEwen ¹², who has conducted pioneering research in the psychology and biology of stress since the late 1960s at Rockefeller University. When a combination of internal and external warning signals (the allostatic load) reaches a critical point, we flip more easily into fight-or-flight mode, and perceive this as "stress". Jan Moynihan, at the University of Rochester, takes this one step further to note that when we routinely operate under high allostatic load, we see disruptions in mood and spirit ¹³. It's important to note that it is the sum of internal and external input that serves as the basis for calculating allostatic load: and while we may not always be able to control our external environments, we can support healthy levels of tension internally using aromatic plants. Our aromatic nervine herbs leave us feeling more calm, warm, present, open and inspired - and this may reduce the internal component of our allostatic load. Events that once were stressful--email, traffic, the disagreeables of life--may be less likely to be perceived that way because the internal landscape is relaxed.

Roses, linden flowers, lemon balm and the citrusy lemongrass, lavender, even mint and the more camphorous mugworts and sages: all these aromatic herbs feature in the traditional herbal record as mood-supportive, stress-relieving allies. It is no wonder they are included in incense and aromatherapy blends. But these herbs can also be taken internally. Neither stimulants nor sedatives, the aromatics bring scent to our inner garden, set the stage for healthy, relaxed tone, and give us another example of the concept of "tonic herb": just as bitters seem to be a premier digestive tonic, supporting good function by engaging our physiologic processes, so too aromatics avoid pushing our moods in one direction or the other, but rather promote good mood and low stress by engaging the limbic system and supporting normal muscle tone in our internal organs. Reach for them first, bring the tonic into your life every day, and navigate stressful times with joy.

1. Rohloff, Jens, et al. "Effect of harvest time and drying method on biomass production, essential oil yield, and quality of peppermint (Mentha× piperita L.)." Journal of agricultural and food chemistry 53.10 (2005): 4143-4148.
2. Meshkatalsadat, Mohammad Hadi, et al. "Identification of essential oil constituents of caraway (Carum carvi) using ultrasonic assist with headspace solid phase microextraction (UA-HS-SPME)." Digest Journal of Nanomaterials and Biostructures 7.2 (2012): 637-640.
3. Karban, Richard, et al. "Kin recognition affects plant communication and defence." Proc. R. Soc. B. Vol. 280. No. 1756. The Royal Society, 2013
4. Mousavi, Seyed AR, et al. "GLUTAMATE RECEPTOR-LIKE genes mediate leaf-to-leaf wound signalling." Nature 500.7463 (2013): 422-426. Mousavi, Seyed AR, Adeline Chauvin, François Pascaud, Stephan Kellenberger, and Edward E. Farmer. "GLUTAMATE RECEPTOR-LIKE genes mediate leaf-to-leaf wound signalling." Nature 500, no. 7463 (2013): 422.
5. Manley, Charles H. "Psychophysiological effect of odor." Critical reviews in food science and nutrition 33.1 (1993): 57-62.
6. There are many clinical investigations of lavender oil inhalation. Some more recent studies: Ziyaeifard, Mohsen, et al. "Evaluation of Lavender Oil Inhalation Effects on Blood Pressure and Heart Rate in Patients Undergoing Coronary Angiography." IRANIAN HEART JOURNAL 18.4 (2017): 29-33.; Allemeier, A., et al. "EFFECTS OF LAVENDER OIL INHALATION ON HEART RATE, SPEED, AND ACCURACY DURING TIMED COGNITIVE TESTING." International Journal of Exercise Science: Conference Proceedings. Vol. 8. No. 5. 2017.
7. Abuhamdah, Sawsan, and Paul L. Chazot. "Lemon Balm and Lavender herbal essential oils: Old and new ways to treat emotional disorders?." Current Anaesthesia & Critical Care19.4 (2008): 221-226.
8. Watanabe, Eri, et al. "Effects of bergamot (Citrus bergamia (Risso) Wright & Arn.) essential oil aromatherapy on mood states, parasympathetic nervous system activity, and salivary cortisol levels in 41 healthy females." Forschende Komplementärmedizin/Research in Complementary Medicine 22.1 (2015): 43-49.
9. Lis-Balchin, Maria, and Stephen Hart. "A preliminary study of the effect of essential oils on skeletal and smooth muscle in vitro." Journal of ethnopharmacology 58.3 (1997): 183-187. Also, see Lis-Balchin, Maria. Aromatherapy science: a guide for healthcare professionals. Pharmaceutical press, 2006.
10. Hills, Judith M., and Philip I. Aaronson. "The mechanism of action of peppermint oil on gastrointestinal smooth muscle: an analysis using patch clamp electrophysiology and isolated tissue pharmacology in rabbit and guinea pig."  Gastroenterology 101.1 (1991): 55-65.
11. Ritter, Malte, et al. "Cardiac and electrophysiological effects of primary and refined extracts from Leonurus cardiaca L.(Ph. Eur.)." Planta medica 76.06: 572-582.
12. McEwen, Bruce S. "Allostasis and allostatic load: implications for neuropsychopharmacology."  -Neuropsychopharmacology  22.2 (2000): 108-124.
13. Across all ages: see O'Connor, Thomas G., Jan A. Moynihan, and Mary T. Caserta. "Annual research review: the neuroinflammation hypothesis for stress and psychopathology in children–developmental psychoneuroimmunology." Journal of Child Psychology and Psychiatry 55.6 (2014): 615-631. and also Kobrosly, Roni W., Edwin van Wijngaarden, Christopher L. Seplaki, Deborah A. Cory-Slechta, and Jan Moynihan. "Depressive symptoms are associated with allostatic load among community-dwelling older adults." Physiology& behavior123 (2014): 223-230.