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The art & science of self-soothing: how to calm your overactive nervous system

Bird's eye view of a geyser
- Oct 03, 2022

Do you ever find it hard to focus, even on projects that interest you? Can you relax without some degree of tension? 

If not, you aren’t alone. Struggles with focus, relaxation, or recovery can often be products of nervous system overactivation. We can become hyper-vigilant for many reasons, even in the absence of pressure in our daily lives. Many stressors are imperceptible, taking a subtle yet persistent toll on the body. 

In this guide, you’ll learn how to calm your overactive nervous system naturally. Rather than just fixing symptoms, these techniques help you soothe stress by sending signals of safety directly to your nervous system. 

As you train your body’s natural stress relief mechanisms, you’ll minimize friction in your daily life and access a clearer, more present-focused version of yourself. 

What is an overactive nervous system?

The autonomic nervous system (ANS) controls all automatic functions in the body. It has two main branches: one energizes us; the other calms us down. The parasympathetic nervous system (PNS), the “rest-and-digest” branch, coordinates regenerative activities like sleep, digestion, and reflection. 

When we’re meditating, sleeping, or relaxing, we’re in a parasympathetic state. These regenerative periods help us maintain balance and give us the resources to handle stress [1]. 

Conversely, the sympathetic nervous system (SNS) coordinates the fight-or-flight response. We detect a stressor: our hearts race, our muscles tighten, and we direct all our resources to addressing the danger [2]. We activate the sympathetic branch with anything that demands a high delivery of blood to the muscles [3]. 

Nervous system imbalance occurs when we struggle to transition from one state to another. It can also happen when one branch gets activated more often. The term "overactive nervous system" refers to sympathetic, rather than parasympathetic, dominance [3]. 

However, it's important to note that the parasympathetic can also become dominant. These imbalances can, in many ways, profoundly disrupt our health. 

An overactive sympathetic nervous system can cause issues like muscle tension, jitters, insomnia, and more. It can also lead to hyper-vigilance, the tendency to detect danger, even in its absence. By contrast, excessive tiredness, depression, and an overactive gag reflex can signal parasympathetic dominance [3]. 

Why the modern world confuses your nervous system

The drivers of nervous system imbalance come in many forms. The sympathetic response evolved to deal with urgent, life-changing emergencies, not everyday events. If your job, finances, or relationships regularly provoke stress, entering fight-or-flight regularly could overwhelm your nervous system. 

But morning traffic or unexpected dental bills are only part of the story. Stress can also run under the radar. Subtle elements of our environment, such as air or light pollution, nutrient deficiencies, or lack of sleep, can also put our bodies into a low-grade yet constant state of high alert. 

Physiologically, stressors impair the body’s capacity to handle everyday activities. It becomes hard to concentrate on our work, for example, if our bodies are reacting as if we’re under threat [1].

When active, the sympathetic system diverts resources away from any process not immediately relevant to defending against a stressor. If your sympathetic response becomes overactive, it can interfere with feedback loops that help maintain homeostasis, like digestion, immune regulation, and other health-supportive processes [4].

Foundations of stress resilience: How to calm an overactive nervous system

The good news: you can train your nervous system to respond differently to stress. There’s a two-way street between our mental states and the nervous system. 

In other words, the way you think influences your nervous system, and the nervous system influences the way you think. You can use heart rate variability (HRV), and by extension, vagal nerve exercises, to calm down during high-stress moments and improve your stress resilience over time.

The link between stress and HRV

HRV is a measure of the dynamics between heartbeats. As you rest, your HRV goes up. During stress or exercise, it goes down. The more complex and varied the dynamics between your heartbeats, the higher your HRV [5]. 

HRV is fundamentally a measure of nervous system balance. If your HRV is high, your body is responding equally to parasympathetic and sympathetic signaling. In other words, you can seamlessly transition from action-ready states into 'rest and digest’ states. Conversely, if your HRV is low, adapting to stress will be more challenging.

Since HRV is a fundamental marker of overall health, people with low HRV often have: 

  • Sleep issues
  • Excessive tiredness
  • Chronic injuries
  • Depression
  • Anxiety
  • Cardiovascular illness
  • Chronic pain [5, 6, 7,8]

Conversely, those with a high HRV find it easier to sleep, focus, and respond to pain [9,10, 11]. Fortunately, identifying and optimizing your HRV is a simple, non-invasive way to balance your nervous system and improve your mood and daily energy levels. 

Vagus nerve exercises, the subject of the rest of this article, improve our stress response because of their effects on HRV.

The science behind vagus nerve stimulation 

The vagus nerve is like a lever we can pull to trigger the body’s relaxation response. Named “vagus” for the Latin word for “wandering, strolling," the vagus is the body’s longest cranial nerve. It begins in the brainstem, extends throughout the body and into the abdomen, communicating messages from the body to the brain, and vice versa. 

As a major part of the parasympathetic system, the vagus nerve regulates automatic processes like breathing, digestion, and heart rate [12].

Crucially, 80% of vagal nerve fibers are afferent, meaning they travel from the body and reach the brain, while the rest are efferent, originating in the brain and extending to the body [13]. In other words, the vagus nerve, in large part, sends messages to the brain about what’s occurring in the body. 

The communication between the vagus nerve and our conscious minds explains gut feelings and other pre-linguistic forms of knowing. Our conscious thoughts or emotions become known to our bodies before our thinking minds. 

Because it turns on the parasympathetic branch, stimulating the vagus nerve can create a near-automatic feeling of calm. As if it were a muscle, we can tone the vagus nerve and improve our stress resilience over time. 

You may want to incorporate a few of the following exercises into your daily routine. These exercises can help during acute stress or even when you want a deeper grounding in the present moment. 

Slow down your breathing

Thanks to feedback loops between the heart, brain, and lungs, the breath is like a lever we can pull to ease our bodies and minds during high-stake moments. Every time we inhale, the heart receives the message to beat faster. 

Conversely, the exhale initiates the "vagal brake," signaling the heart to slow down [14]. You can stimulate your vagus nerve by making your exhales longer than your inhales. 

This mode of breathing changes the degree to which the vagus nerve influences the heart. Slow inhalations allow the vagus to inhibit the heart rate, issuing a message of safety to the brain. As you continue to breathe in this way, the brain will further support this message by activating parasympathetic functions.

Below, we focus on two ways to stimulate the vagus with the breath: diaphragmatic breathing and music, particularly singing and playing wind instruments.

Deep, diaphragmatic breathing

When anxious or stressed, we take quick, shallow breaths, initiating a vicious cycle: one where stressful thoughts encourage shallow breathing, and shallow breathing further contributes to tense, fearful thoughts. But it can be surprisingly easy to intervene in this process, using the long and slow exhales to invoke a sense of calm.

A large body of research has shown that controlled breathing exercises regulate blood pressure, heart rate, and anxiety. Research in Frontiers in Human Neuroscience focused on the ways slowed breathing and extended exhales stimulate the vagus, causing parasympathetic dominance [15]. Contemplative practices, such as pranayama yoga [15], Kundalini yoga [16], and Yoga Nidra [17] trigger calmness because of their emphasis on controlled breathing.

The simplest way to change the dynamics of your breathing is to do a 5-10 minute breath-focused meditation. 

Set a timer, then start by taking a deep breath in, keeping your hand on your stomach to feel it expand as you inhale. Hold the inhale for 5-10 seconds, extending your exhale for about twice as long. If your attention drifts (and it will), let the rising and falling of your hand on your stomach serve as an anchor to the present moment. 

Music: Self-expression, social bonding, and finding your flow

When you sing, your exhales extend longer than your inhales [14]. As a result, you adopt a breathing rhythm that sends the body safety signals. Simultaneously, singing, humming, chanting, and playing wind instruments (like the flute) trigger muscles in the back of the throat connected to the vagus nerve, engaging the parasympathetic system.

Researchers have long been fascinated by the health benefits of music. For example, a study in Frontiers in Human Neuroscience explored the neurochemical events underlying group singing, finding reductions in adrenocorticotropic hormone (ACTH), which controls the production of the stress hormone, cortisol. 

The research also showed that improvised group singing raised oxytocin, which drives social bonding [18]. These findings suggested that making music with a group can not only reduce stress and anxiety but inspire what they call "social flow," a feeling of interconnectedness with other people. 

This state of interconnectedness aligns with the concept of flow, known to psychologists as a state of total absorption in the present. The flow state is a peak experience where a person feels so at peace with a task that the process becomes its own reward. 

Because it turns down the volume on self-criticism and other types of inner chatter, flow can bring a high level of clarity and fulfillment to your life. If music isn’t your thing, consider committing to a daily practice of whatever brings you joy. Like the question author Brené Brown posed in her book Daring Greatly, ask yourself, “What’s worth doing, even if I fail?”

Hormesis: not all stress is bad

"Hormesis" refers to the idea that short bursts of certain types of stressors trigger cellular events that support health. In large amounts, stressors like high-intensity exercise, heat, or cold water immersion can harm you. 

But when done intermittently and in small quantities, they can make you more resilient to mental and physiological stress in the future [19]. 

Although hormetic stressors come in many forms, extreme temperature exposure, high-intensity interval training (HIIT), and weight training have intriguing effects on the nervous system. 

Cold and heat exposure 

Cold exposure has become a personal development cliché. But you do not need to swim in the arctic or bathe in ice to reap its benefits. Even 30 seconds of cold exposure after a warm shower can train the body to respond differently to stress. 

Cold water first provokes a sympathetic response, usually marked by a rapid, shallow inhale. But as you acclimate, you activate the parasympathetic response, with calming effects lasting hours after exposure [20]. Cold immersion also stimulates the hormone and neurotransmitter norepinephrine, supporting mental clarity, focus, and a positive mood [21]. 

If you’re more comfortable with heat, saunas can also train your parasympathetic response. A 2019 study in Complementary Therapies in Medicine found that 30-minute sauna sessions significantly affected autonomic nervous system balance [22]. 

During the cool-down period, HRV increased, signaling a decrease in sympathetic function and a rise in parasympathetic function. If you become a sauna bather, start with 5 to 10 minutes, then gradually increase your time inside. 

High-Intensity Interval Training 

Not all types of exercise support nervous system balance. If you’re already stressed, certain types of exercise can exacerbate stress by provoking a sympathetic response. You can exercise to relieve (rather than amplify) stress by optimizing its duration and intensity. 

Research in the Journal of Science and Medicine in Sport found that short bursts of high-intensity exercise, followed by periods of less rigorous movement, were most supportive of neuroplasticity [23]. Neuroplasticity, or the capacity of the brain to change its structure and function to accommodate changing conditions, is essential for learning, memory, and injury recovery.

HIIT may support neuroplasticity because of the way it affects stress hormones. The authors theorized that high-intensity interval training (HIIT) helps the brain regulate cortisol production during exercise. 

Exercise tends to drive cortisol production, but training at a high and continuous intensity may raise levels to the extent that they interfere with the beneficial effects of exercise [23]. By pairing bouts of high-intensity with slower, less demanding intervals, cortisol levels don't get high enough to interfere with neuroplasticity gains. 

An example of cardio-focused HIIT training might consist of one minute of rigorous, fast-paced exercise, followed by a two-minute slowdown period, continuing with this alternating pattern for about 15 minutes. HIIT can also be a way of enjoying the benefits of high-intensity exercise without the time commitment. 

Weight training

Weight training equips the nervous system with proprioceptive input, providing more data about the environment. Proprioceptive input allows you to brush your teeth without a mirror or walk outside without running into street signs or other people. Issues with proprioception can manifest as clumsiness, balance problems, or issues with maintaining good posture. 

Exercises that provide proprioceptive input can calm the nervous system because it won’t need to work as hard to make sense of its environment.

Interestingly, weight lifting impacts the nervous system before the muscles. A study in the Journal of Neuroscience showed that weight training stimulates the nervous system through the reticulospinal tract, which controls movement and posture [24].

 The authors found that weight training improves strength because of increased neural input to the muscles, meaning that we become stronger as a downstream effect of strengthening the nervous system. Of course, as you strengthen your nervous system, you also support its ability to bounce back from stress.

Build strong support networks 

In solitude, we can do a lot to balance our nervous systems. Yet the importance of social ties can’t be overstated. From an evolutionary standpoint, humans evolved to collect information about ourselves and the environment from vocal intonation and facial expression. 

At birth, babies cannot thrive (and in some cases, survive) without physical contact and support from a caretaker [25]. This need doesn’t disappear as a person ages. 

According to research in the American Psychologist Journal, the presence of others reduces our perception of pain. It even changes how we perceive stress, weakening neural activity in brain regions associated with threat detection. 

Researchers suggest that chemical messengers we produce internally, such as opioids or oxytocin, may drive these effects [26]. Another benefit of friendships, laughter, can also stimulate the vagus nerve. Multiple studies have shown that laughter increases HRV thanks to changes in breathing patterns [27].

Social interactions are a primary way humans regulate their emotional states. Interestingly, social cues, such as vocal intonation and facial expression, calm the nervous system by conveying insights about the feelings and intentions of the other person. 

According to research by neuroscientist Dr. Stephen Porges, the vagus nerve co-evolved with our facial and head muscles [14]. As a result of this co-evolution, facial expressions and tone of voice strongly influence whether we perceive someone as a friend or a threat to our safety.

This information exchange is why being in another's presence can feel more relieving than texting, emailing, or even speaking over video. While texting and email exchange serve valuable functions, they provide only a fraction of the data the nervous system needs to feel completely at ease. 

Touch like your health depends on it  

Whether physical touch is your love language or you’re not exactly a big hugger, change can change our physiologies in measurable ways. Touch can stimulate the release of feel-good hormones like dopamine, serotonin, and oxytocin, which positively impact mood [25].

The Apollo wearable brings balance to the nervous system through the power of touch. By mimicking the oscillation patterns between the heart and lungs during deep breathing, the Apollo wearable helps to relieve pressure in stressful situations.

In one double-blind randomized placebo-controlled trial [28], researchers tested whether Apollo improved mental performance during an assessment of cognitive function called the Paced Auditory Serial Addition Test (PASAT). The results were compelling.

Mirroring the effects of boredom and frustration on concentration levels, the PASAT purposefully induces stress. However, among subjects who used Apollo during the test, their HRV improved by two to three times over average within just three minutes. 

These results showed that the Apollo wearable helped reduce the consequences of the fight-or-flight response, restoring balance in a high-pressure situation.

Apart from using Apollo and giving hugs or massages, cozying up to a pet can be another way to meet the need for touch. This systematic review found that pet ownership had mental health benefits, with stroking and close physical contact improving mood. 

Significantly, pet ownership also lessened worry and social isolation, especially for people with conditions worsened by stress. One interesting reason for these effects was that walking a pet led to more spontaneous conversations with strangers in public.

Practice gratitude

Finally, one of the simplest ways to balance the nervous system may also be the most elegant. Most people feel better after writing down what they feel grateful about. But its benefits run deeper: gratitude can even change how your genes and cells function. 

A 2016 study in Psychosomatic Medicine studied the effects of gratitude journaling for people in the early stages of heart failure. The researchers found that the practice reduced markers of inflammation and increased HRV [33]. 

Additionally, research has shown that gratitude meditation can help with emotion regulation and self-motivation in a way that resembles meditation. Like mindfulness, gratitude journaling improves connectivity among brain regions responsible for motivation and reward processing [34]. 

While there are many journaling apps, check out 750words.com, a journaling website that rewards you with streaks for daily writing. It can also be an excellent tool for self-analysis. You can see reports of the moods, subjects, and emotions that most commonly appear in your writing. 

To support your nervous system, remember your roots

In our productivity-focused society, it’s easy to forget that we are social animals. Fundamentally, the pursuit of safety is behind everything we do (14). 

On occasion, our more primitive needs can seem to work against our desire to produce, create, or achieve. But sometimes it’s our very refusal to step away from work, spend time with friends, or honor rest periods that detail our well-being the most. 

To recap, you can send signals of safety to your nervous system through:

  • Deep breathing 
  • Hormetic stressors 
  • Soothing touch 
  • Social connections 
  • Gratitude 

Rather than changing your state with herbal tea, stress relief supplements, (or trying to reason yourself out of stress), the best way to soothe your nervous system is to speak its language. 

Because the vagus nerve reliably engages our parasympathetic system, activities that stimulate it are the most holistic and far-reaching ways to soothe yourself.

 

 

Sources:

  1. Berntson GG, Quigley K, Lozano DL. Cardiovascular Psychophysiology. In: Cacioppo JT, Tassinary LG, Berntson GG, editors. Handbook of psychophysiology. Fourth edition. ed. Cambridge, United Kingdom; New York, NY, USA: Cambridge University Press; 2017. p. xvi, 715 pages.
  2. Waxenbaum, JA, Reddy, V, Varacallo, M. Anatomy. Autonomic nervous system. StatPearls. [Updated 2021 Jul 29]. https://www.ncbi.nlm.nih.gov/books/NBK539845/
  3. Dr. Chad. (2012, February, 12). Using Exercise to  Balance the Nervous System. Natural Path Health Center. https://naturalpathhealthcenter.com/2012/02/14/using-exercise-to-balance-the-nervous-system/
  4. Schneiderman, N, Ironson, G, Siegel, SD. Stress and health: Psychological, behavioral, and biological determinants. Annual Review of Clinical Psychology. 2008. 1: 607-628. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2568977/
  5. Lehrer PM, Gevirtz R. Heart rate variability biofeedback: how and why does it work? Frontiers in Psychology. 2014; 5: 756. doi: 10.3389/fpsyg.2014.00756. 
  6. Dodds KL, Miller CB, Kyle SD, Marshall NS, Gordon CJ. Heart rate variability in insomnia patients: A critical review of the literature. Sleep Med Rev. 2017 Jun;33:88-100. doi:10.1016/j.smrv.2016.06.004. Epub 2016 Jun 28. Review. 
  7. Dennis PA, Kimbrel NA, Sherwood A, Calhoun PS, Watkins LL, Dennis MF, Beckham JC. Trauma and Autonomic Dysregulation: Episodic-Versus Systemic-Negative Affect Underlying Cardiovascular Risk in Posttraumatic Stress Disorder. Psychosom Med. 2017 Jun;79(5):496-505. Doi: 10.1097/PSY.0000000000000438. 
  8. Dennis PA, Dedert EA, Van Voorhees EE, Watkins LL, Hayano J, Calhoun PS, Sherwood A, Dennis MF, Beckham JC. Examining the Crux of Autonomic Dysfunction in Posttraumatic Stress Disorder: Whether Chronic or Situational Distress Underlies Elevated Heart Rate and Attenuated Heart Rate Variability. Psychosomatic Medicine. 2016 Sep;78(7):805-9. Doi: 10.1097/PSY.0000000000000326.
  9. Tsai HJ, Kuo TB, Lee GS, Yang CC. Efficacy of paced breathing for insomnia: enhances vagal activity and improves sleep quality. Psychophysiology. 2015 Mar;52(3):388-96. doi: 10.1111/psyp.12333. 
  10. Lumley MA, Cohen JL, Borszcz GS, et al. Pain and Emotion: A Biopsychosocial Review of Recent Research. Journal of Clinical Psychology. 2011;67(9):942-968. doi:10.1002/jclp.20816
  11. Koenig J, Loerbroks A, Jarczok MN, Fischer JE, Thayer JF. Chronic Pain and Heart Rate Variability in a Cross-Sectional Occupational Sample: Evidence for Impaired Vagal Control. Clin J Pain. 2016. Mar;32(3):218-25. doi: 10.1097/AJP.0000000000000242. PubMed PMID: 25924095
  12. Porges, SW, Doussard-Roosevelt, Maiti, AK. Vagal tone and the physiological regulation of emotion. Monographs of the Society for Research in Child Development. 1994. 59(2/3): 167-186. Doi: https://doi.org/10.2307/1166144
  13. Breit, S, Kupferberg, A, Rogler, G, and Hasler, G. Vagus Nerve as Modulator of the Brain-Gut Axis in Psychiatric and Inflammatory Disorders. Frontiers in Psychiatry. 2018, 9(44). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5859128/
  14.  Porges, SW. (2017). The pocket guide to the polyvagal theory: The transformative power of feeling safe. W.W. Norton & Company.
  15. Gerritsen, Roderik, J.S., Band, Guido, PH. Breath of life: The respiratory vagal stimulation model of contemplative activity. Frontiers in Human Neuroscience.2018, 12(397). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6189422/
  16. Goshvarpour, Ateke & Goshvarpour, Atefeh. Comparison of higher order spectra in heart rate signals during two techniques of meditation: Chi and Kundalini meditation. Cognitive Neurodynamics. 2012. 7(39-46). https://link.springer.com/article/10.1007/s11571-012-9215-z
  17. Ferreira-Vorkapic, Borba-Pinheiro, CJ, Marchioro, M, Santana, D. The impact of Yoga Nidra and seated meditation on the mental health of college professors. International Journal of Yoga. 2018. 11(3):215-223. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6134749/
  18. Keeler, J.R., Roth, E.A., Neuser, BL. et al. The neurochemistry and social flow of singing: bonding and oxytocin. Frontiers in Human Neuroscience. 2015. 23. https://www.frontiersin.org/articles/10.3389/fnhum.2015.00518/full
  19. Mattson, MP. Hormesis defined. Ageing Research Reviews. 2008. 7(1):1-7. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2248601/
  20. Jungmann, M. Vencatachellum, S, Van Ryckeghem, DV, Vogele, C. Effects of Cold Stimulation on Cardiac-Vagal Activation in Healthy Participants: Randomized Controlled Trial. 2018. 2(2):e10257. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6334714/
  21. Nicola, Louisa. Neural (2021, August 31). Neural effects of cold immersion. Neuro Athletics. https://neuroathletics.substack.com/p/neural-effects-of-cold-immersion?s=r
  22. Laukkanen, T. Lipponen, J. Kunutsor, SK. et al. Recovery from sauna bathing favorably modulates cardiac autonomic nervous system. Complementary Therapies in Medicine. 2019. 45: 190-197. https://www.sciencedirect.com/science/article/abs/pii/S0965229919301943
  23. Mellow, ML, Goldsworthy, MR, Coussens, S. Smith, AE. Acute aerobic exercise and neuroplasticity of the motor cortex: A systematic review. https://www.jsams.org/article/S1440-2440(19)30799-6/fulltext 
  24. Glover, IS & Baker, SN. Cortical, Corticospinal, and Reticulospinal Contributions to Strength Training. Journal of Neuroscience. 2020. 40(30):5820-5832. https://www.jneurosci.org/content/40/30/5820
  25. Young, J, Pritchard, R. Bottle, C, Banwell, H. Pets, touch, and COVID-19: Health benefits from non-human touch through times of stress. Journal of Behavioral Economics for Policy. 2020. COVID-19 Special Issue 2, 25-33. https://sabeconomics.org/wordpress/wp-content/uploads/JBEP-4-S2-3.pdf
  26. Pietromonaco, PR & Collins, NL. Interpersonal mechanisms linking close relationships to health. American Psychologist. 2017. 72(6):531-542. https://pubmed.ncbi.nlm.nih.gov/28880100/
  27. Dolgoff-Kasper, R, Baldwin, A, Johnson, S, Edling,  N, Sethi, GK. Effect of laughter on mood and heart rate variability in patients awaiting organ transplantation: a pilot study. Alternative Therapies in Health and Medicine. 2012. 18(4):53-58 https://pubmed.ncbi.nlm.nih.gov/22891377/
  28. Clinical study validates that Apollo improves cognitive performance and heart rate variability (HRV). (2020, January, 20). Apollo Neuro. https://apolloneuro.com/blogs/news/clinical-study-validates-that-apollo-improves-cognitive-performance-and-heart-rate-variability-hrv
  29. Brooks, HL, Rushton, K, Lovell, K, Bee, P, et al. The power of support from companion animals for people living with mental health problems: a systematic review and narrative synthesis of the evidence. BMC Psychiatry. 2018. 18(31) https://link.springer.com/content/pdf/10.1186/s12888-018-1613-2.pdf
  30. What are circadian rhythms? National Institute of General Medical Sciences. https://nigms.nih.gov/education/fact-sheets/Pages/Circadian-Rhythms.aspx
  31. Brainard, J, Gobel, M, Scott, B, Koeppen, Eckle, T. Health implications of disrupted circadian rhythms and the potential for daylight as therapy. Anesthesiology. 2015. 122(5):1170-1175. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4632990/
  32. Tahkamo, L, Partonen, T, Pesonen, A. Systematic review of light exposure impact on human circadian rhythm. Chronobiology International. 2019. 36(2):151-170. https://pubmed.ncbi.nlm.nih.gov/30311830/
  33. Redwine, L, Henry, BL, Pung, MA, Wilson, K. et al. A pilot randomized study of a gratitude journaling intervention on HRV and inflammatory biomarkers in Stage B heart failure patients. Psychosomatic Medicine. 2016. 78(6): 667-676. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4927423/
  34. Kyeong, S, Kim, J, Jin Kim, D., Kim HE, Kim, J. Effects of gratitude meditation on neural network functional connectivity and brain-heart coupling. Scientific Reports. 2017. 7:5058. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5506019/