The Parasympathetic Nervous System:

Rest, Digest, and the Body's Path Back to Balance

 

Breath slows, heart finds peace

Gut moves in quiet rhythm —

Body rests complete

 

After battle fades

The body reclaims stillness —

Nerves unwind their grip

 

by CEJames (researcher/author) & Akira Ichinose (editor/research assistant)

 

DISCLAIMER

The content presented here is for educational and entertainment purposes only and does not constitute legal advice or a certified self-defense methodology. Laws governing the use of force vary by jurisdiction. Readers should consult a qualified attorney and seek instruction from a certified self-defense professional before making any decisions regarding personal protection.

 

Introduction

Your body is always listening. Every heartbeat, every breath, every ripple of digestion is being quietly managed by a system most people never think about — the autonomic nervous system. Split into two main branches, it runs your internal world on autopilot. One branch hits the gas when danger appears; the other taps the brakes and says, relax, we made it. That second branch is the parasympathetic nervous system, and it is the unsung architect of recovery, healing, and calm.

Nicknamed the "rest and digest" system, the parasympathetic branch counterbalances the well-known "fight or flight" response driven by its partner, the sympathetic nervous system. Where the sympathetic state floods you with adrenaline and redirects blood to your muscles, the parasympathetic state does the opposite — it slows your heart rate, stimulates digestion, promotes cellular repair, and returns your body to a state of equilibrium. Understanding how this system works is not merely academic. For martial artists, veterans, athletes, and anyone navigating high-stress modern life, knowing how to consciously activate this system is a practical survival skill.

What Is the Parasympathetic Nervous System?

The autonomic nervous system (ANS) governs bodily functions that operate below conscious awareness — heart rate, blood pressure, respiration, digestion, and glandular secretion. It is divided into three divisions: sympathetic, parasympathetic, and enteric (the gut-brain connection). The sympathetic and parasympathetic divisions work in dynamic opposition, ☯️ each modulating the other to keep the body in homeostasis — a state of internal balance suited to the demands of the moment (Jänig, 2022).

The parasympathetic nervous system originates in two anatomically distinct regions: the brainstem (cranial nerves III, VII, IX, and X) and the sacral spinal cord (segments S2–S4). Its nerve fibers use acetylcholine as their primary neurotransmitter, binding to muscarinic receptors throughout the body to produce effects that are broadly restorative and energy-conserving. In contrast to sympathetic fibers, which have short preganglionic and long postganglionic neurons, parasympathetic fibers have long preganglionic neurons that synapse in ganglia located very close to — or within — the target organs themselves, giving the system fine-grained, localized control (Bear et al., 2020).

How the Rest and Digest Response Works

When a threat has passed — or when the brain perceives safety — the parasympathetic system shifts into dominance. Think of it as the body's transition from "battle mode" back to "base camp." Several simultaneous processes begin almost immediately.

The heart rate slows. The sinoatrial node, the heart's natural pacemaker, receives inhibitory signals via the vagus nerve, reducing the number of beats per minute and lowering overall cardiac output. This is why slow, deliberate breathing can measurably lower your pulse within seconds — you are directly engaging parasympathetic tone (Thayer & Lane, 2009).

Blood flow is redistributed. During sympathetic arousal, blood is shunted away from the digestive organs and toward skeletal muscles. As parasympathetic tone rises, blood returns to the gastrointestinal tract. Peristalsis — the wave-like muscular contractions that move food through the intestines — resumes. Digestive enzymes are secreted. The liver ramps up glycogen storage. The whole apparatus of nutrient processing restarts (Marieb & Hoehn, 2019).

The pupils constrict. Parasympathetic fibers from cranial nerve III cause the pupils to narrow, reducing light intake — the opposite of the wide-eyed, high-alert state of sympathetic activation. Salivary glands also become more active (those dry-mouth moments under stress are a classic sympathetic effect; the return of saliva is your parasympathetic system signaling that the danger has passed).

Cellular repair accelerates. Growth hormone secretion, immune function, and tissue regeneration are all enhanced during parasympathetic-dominant states — particularly during sleep, which is the body's most powerful rest-and-digest window. Chronic sympathetic dominance suppresses immune surveillance; chronic parasympathetic access restores it (Besedovsky et al., 2019).

The Vagus Nerve: The Highway of Calm

No single structure is more central to the parasympathetic system than the vagus nerve — cranial nerve X. "Vagus" is Latin for "wandering," and the name is apt. This nerve meanders from the brainstem down through the neck, chest, and abdomen, sending parasympathetic signals to the heart, lungs, stomach, liver, pancreas, gallbladder, small intestine, and colon. It is the longest cranial nerve in the body and carries roughly 80% of its fibers in an afferent direction — meaning most of the vagus nerve's traffic runs from the body back up to the brain, not the other way around (Porges, 2011).

This bottom-up architecture matters enormously. It means the brain is, in large measure, reading the body's state rather than dictating it. When the body sends signals of safety — slow breathing, relaxed diaphragm, a calm gut — the brain registers safety and sustains parasympathetic tone. Conversely, when breathing is rapid and shallow and the gut is tense, the brain interprets threat even in the absence of actual danger. This is why body-based interventions — controlled breathing, posture, physical warmth — can shift your emotional state faster than purely cognitive approaches (Porges, 2011).

Vagal tone — the background level of vagus nerve activity — is measurable through heart rate variability (HRV). High HRV indicates a flexible, responsive autonomic system with strong parasympathetic capacity. Low HRV correlates with chronic stress, cardiovascular disease, depression, and poor stress resilience. Training vagal tone through consistent practices like slow breathing, cold exposure, and meditation has become a focus of serious clinical research (Thayer & Lane, 2009).

The Enteric Connection: Your Second Brain

The gastrointestinal tract contains its own intrinsic nervous system — the enteric nervous system — sometimes called the "second brain." It houses roughly 500 million neurons and operates with significant autonomy from the central nervous system, though it communicates extensively with the brain via the vagus nerve. The parasympathetic system is the principal modulator of enteric activity. When rest-and-digest is engaged, gut motility increases, sphincters relax appropriately, and the complex machinery of digestion and nutrient absorption runs at full capacity (Furness, 2012).

The gut-brain axis — the bidirectional communication network linking the enteric and central nervous systems — is also a major pathway for emotional regulation. The enteric nervous system produces approximately 95% of the body's serotonin. Disruption of parasympathetic tone (chronic stress, trauma) impairs gut function, alters serotonin metabolism, and contributes to conditions ranging from irritable bowel syndrome to anxiety and depression. Conversely, restoring parasympathetic access improves both gastrointestinal health and psychological resilience (Mayer, 2016).

Sympathetic vs. Parasympathetic: The Balance That Matters

Neither the sympathetic nor the parasympathetic system is inherently good or bad. Both are essential. The sympathetic system gets you through the threat; the parasympathetic system rebuilds you after it. The problem arises when the balance is chronically skewed. Modern life — perpetual deadlines, information overload, financial pressure, unresolved conflict — can keep the sympathetic system in a state of low-grade activation that never fully resolves. The body stays primed, cortisol remains elevated, inflammatory markers creep upward, and the recuperative functions of the parasympathetic system are perpetually shortchanged (Jänig, 2022).

The long-term consequences of sympathetic dominance include hypertension, impaired digestion, suppressed immunity, disrupted sleep, impaired memory consolidation, and elevated risk of cardiovascular disease. These are not abstract risks — they are the physiological cost of a nervous system that never gets to stand down. Building deliberate parasympathetic access into daily life is not indulgence; it is physiological maintenance (Marieb & Hoehn, 2019).

Practical Ways to Activate the Rest and Digest System

The good news is that the parasympathetic system is highly accessible through intentional practice. Several techniques have robust research support.

Slow, diaphragmatic breathing is the most powerful and immediate lever. Extending the exhalation relative to the inhalation — such as a four-count inhale and an eight-count exhale — directly stimulates vagal afferents and shifts autonomic balance toward parasympathetic dominance within minutes. Resonance frequency breathing, typically around 4.5 to 6 breaths per minute, maximizes HRV and parasympathetic tone (Zaccaro et al., 2018).

Cold water exposure has demonstrated consistent vagal tone enhancement. Facial immersion in cold water activates the diving reflex — a powerful parasympathetic response mediated largely by the vagus — producing rapid heart rate deceleration. Regular cold exposure, even modest cold showers, appears to progressively improve resting vagal tone (Jungmann et al., 2018).

Mindfulness meditation, practiced consistently, measurably increases HRV and parasympathetic activity. The mechanism is partly attentional — reducing anticipatory threat processing — and partly physiological, as mindful breathing and body-scan practices directly engage the interoceptive pathways of the vagus nerve (Krygier et al., 2013).

Physical warmth promotes parasympathetic tone. Warm baths, saunas, and even warm beverages activate thermoreceptors connected to vagal pathways. Social connection and safe, affiliative touch — shown to release oxytocin — also enhance parasympathetic activity, which is consistent with Polyvagal Theory's emphasis on the nervous system's orientation toward social engagement as the highest-order safety signal (Porges, 2011).

Sleep is the body's most profound rest-and-digest state. Slow-wave and REM sleep are both characterized by strong parasympathetic dominance. Protecting sleep — its duration, regularity, and quality — is the single highest-leverage parasympathetic intervention available. Consistent sleep hygiene, reduced pre-sleep screen exposure, and a cool, dark sleeping environment all support the parasympathetic conditions under which physical and psychological recovery occur most fully (Walker, 2017).

Why This Matters for the Martial Artist and the Veteran

For those whose training or service regularly places them in states of high physiological arousal — whether on the dojo floor or in a combat theater — the ability to access the parasympathetic system quickly and reliably is a core competency, not a luxury.

In Okinawan martial arts traditions, concepts like kokyu (breath control) and mushin (no-mind) are partly descriptions of parasympathetic access under stress. The practitioner who can slow their heart rate between exchanges, maintain gut-brain clarity under pressure, and return rapidly to baseline after an intense training round has a measurable physiological advantage over one who cannot. The old masters may not have had the language of the autonomic nervous system, but they built parasympathetic training into kata practice, breathing exercises, and meditative stillness because they understood its effects empirically.

For veterans dealing with the chronic sympathetic arousal that often persists after service, understanding this system offers both explanation and remedy. The nervous system is not broken — it adapted, correctly, to conditions that demanded constant vigilance. Bringing it back into balance requires patient, consistent engagement of the parasympathetic pathways through the practices described above. It is, in the most literal sense, a return to self.

Conclusion

The parasympathetic nervous system is the body's master restoration system. Through the wandering pathways of the vagus nerve, it orchestrates a symphony of recovery — slowing the heart, feeding the gut, clearing the mind, and rebuilding the tissues that stress has taxed. It operates largely beneath awareness, but it is entirely accessible to those who know how to reach it.

The rest-and-digest response is not weakness. It is the biological foundation of resilience. A warrior who cannot stand down is a warrior who burns out. A practitioner who can cycle efficiently between activation and recovery — between intensity and stillness — is one whose body remains a reliable instrument across decades of practice and life. Cultivate the parasympathetic system with the same discipline you bring to any other aspect of training. The returns are physiological, psychological, and philosophical.

 

References

Bear, M. F., Connors, B. W., & Paradiso, M. A. (2020). Neuroscience: Exploring the brain (4th ed.). Jones & Bartlett Learning.

Besedovsky, L., Lange, T., & Haack, M. (2019). The sleep-immune crosstalk in health and disease. Physiological Reviews, 99(3), 1325–1380. https://doi.org/10.1152/physrev.00010.2018

Furness, J. B. (2012). The enteric nervous system and neurogastroenterology. Nature Reviews Gastroenterology & Hepatology, 9(5), 286–294. https://doi.org/10.1038/nrgastro.2012.32

Jänig, W. (2022). Autonomic nervous system. In G. J. Bhatt (Ed.), Physiology of the autonomic nervous system (3rd ed., pp. 1–45). Springer.

Jungmann, M., Vencatachellum, S., Van Ryckeghem, D., & Vögele, C. (2018). Effects of cold stimulation on cardiac-vagal activation in healthy participants. PLOS ONE, 13(10), e0204731. https://doi.org/10.1371/journal.pone.0204731

Krygier, J. R., Heathers, J. A. J., Shahrestani, S., Abbott, M., Gross, J. J., & Kemp, A. H. (2013). Mindfulness meditation, well-being, and heart rate variability. International Journal of Psychophysiology, 89(3), 305–313. https://doi.org/10.1016/j.ijpsycho.2013.06.017

Marieb, E. N., & Hoehn, K. (2019). Human anatomy & physiology (11th ed.). Pearson Education.

Mayer, E. A. (2016). The mind-gut connection: How the hidden conversation within our bodies impacts our mood, our choices, and our overall health. Harper Wave.

Porges, S. W. (2011). The polyvagal theory: Neurophysiological foundations of emotions, attachment, communication, and self-regulation. W. W. Norton & Company.

Thayer, J. F., & Lane, R. D. (2009). Claude Bernard and the heart-brain connection: Further elaboration of a model of neurovisceral integration. Neuroscience & Biobehavioral Reviews, 33(2), 81–88. https://doi.org/10.1016/j.neubiorev.2008.08.004

Walker, M. (2017). Why we sleep: Unlocking the power of sleep and dreams. Scribner.

Zaccaro, A., Piarulli, A., Laurino, M., Garbella, E., Menicucci, D., Neri, B., & Gemignani, A. (2018). How breath-control can change your life: A systematic review on psycho-physiological correlates of slow breathing. Frontiers in Human Neuroscience, 12, 353. https://doi.org/10.3389/fnhum.2018.00353

Breathing Through the Storm

Managing the Adrenal Response Under Pressure of Conflict and Violence

Breath slows the red tide —

the body learns what the mind

forgot it once knew.

Four counts in, hold still —

the fist unclenches when the

exhale touches peace.

by CEJames (researcher/author) & Akira Ichinose (editor/research assistant)

 

DISCLAIMER

The content presented here is for educational and entertainment purposes only and does not constitute legal advice or a certified self-defense methodology. Laws governing the use of force vary by jurisdiction. Readers should consult a qualified attorney and seek instruction from a certified self-defense professional before making any decisions regarding personal protection.

 

The Chemical Avalanche: What Your Body Does When Danger Arrives

Before we can talk about breathing, we need to talk about what we are breathing against. When your brain perceives a genuine threat — a raised fist, a blade clearing a pocket, a shove that signals things are about to get very bad — it does not wait for your considered opinion. The hypothalamus fires a signal to the adrenal glands, those little triangular caps sitting atop your kidneys, and within seconds you are swimming in epinephrine, what most of us just call adrenaline. Norepinephrine piles on right behind it, and cortisol comes along for the extended ride. This is the sympathetic nervous system throwing every emergency lever it has.

What happens in your body is nothing short of a physiological reorganization. Your heart rate climbs — sometimes explosively. Bruce Siddle, who spent years studying stress and human performance in law enforcement, documented that fine motor skills begin degrading at heart rates above 115 beats per minute, complex motor skills suffer above 145 bpm, and above roughly 175 bpm you can experience perceptual tunnel vision, auditory exclusion, and even cognitive irrational behavior (Siddle, 1995). The blood is being shunted away from the extremities and toward the large muscle groups. Your pupils dilate. Digestion stops. Your breathing — and here is where this conversation really begins — becomes rapid, shallow, and driven by the chest rather than the diaphragm.

That last part is the problem and the solution, all at once. Because breathing, unlike your heart rate or your cortisol levels, is the one branch of the autonomic nervous system that you can consciously override. You cannot decide your adrenal glands will stop pumping. You can decide how you breathe, and that decision reaches straight into the nervous system and starts pulling things back toward equilibrium.

The Vagal Brake: Why Breathing Is Your Best Available Tool

The vagus nerve is the longest cranial nerve in the body, running from the brainstem down through the neck, the chest, and into the abdomen. It is the primary highway of the parasympathetic nervous system — the "rest and digest" counterpart to the sympathetic "fight or flight" machinery. When you breathe slowly and deeply, particularly on the exhale, you stimulate the vagus nerve in ways that produce what researchers call increased vagal tone. The heart rate slows. Blood pressure drops modestly. The prefrontal cortex — the part of your brain responsible for rational decision-making — gets better blood flow and starts coming back online (Porges, 2011).

This is not mysticism. It is not a marketing slogan. It is measurable, repeatable physiology. The diaphragm contains mechanoreceptors — pressure-sensitive nerve endings — that send signals directly to the brainstem when the muscle moves through its full range. When you breathe shallowly and rapidly from the chest, those receptors go quiet. The brainstem interprets ongoing shallow breathing as confirmation that the emergency is real and continuing, which keeps the sympathetic flood going. When you consciously drive the breath down into the belly, you are sending a different message entirely.

Tactical Breathing: The Box and What Lives Inside It

The most widely taught breathing protocol in high-stress professions — law enforcement, military special operations, emergency medicine — goes by several names: box breathing, combat breathing, or the four-count tactical breath. Dave Grossman and Loren Christensen, in On Combat, describe it plainly: inhale for a count of four, hold for a count of four, exhale for a count of four, hold for a count of four (Grossman & Christensen, 2008). That four-sided structure is why it is called a box.

Let us walk through what actually happens inside each of those four counts, because understanding the mechanism makes you a better practitioner.

The Inhale (Count of Four)

Breathe in through the nose, not the mouth. Nasal breathing filters and warms the air, but more importantly, it slightly increases resistance, which slows the breath and promotes fuller diaphragmatic movement. As you inhale, your belly should expand first — push it out, not in. This tells you the diaphragm is moving downward and doing the actual work of pulling air into the lower lobes of the lungs, where the most efficient gas exchange happens. The chest rises secondarily. Counting to four in your head at a moderate pace anchors the duration and gives the racing mind something concrete to do, which is a small cognitive interruption to the panic loop.

The Hold (Count of Four — Post-Inhale)

Holding the breath at full inhale allows the oxygen already drawn into the lungs to continue crossing into the bloodstream, maximizing the efficiency of that breath. More practically, the act of holding requires a conscious muscular effort — you are gently engaging the diaphragm and the intercostal muscles in a way that keeps you anchored in your body rather than dissociating into the threat. Studies on respiratory sinus arrhythmia — the natural variation in heart rate tied to the breathing cycle — show that deliberate breath-hold phases help stabilize that rhythmic variation, which is itself a marker of autonomic balance (Lehrer et al., 2000).

The Exhale (Count of Four)

The exhale is where most of the parasympathetic magic lives. Extending the exhale — even making it slightly longer than the inhale when you have the latitude to do so — has a disproportionately powerful vagal effect. The exhale phase is when the heart rate actually slows. Breathe out through slightly pursed lips, or simply let the air leave in a controlled, even stream. You do not need to force every molecule of air out. Just let the belly fall, the diaphragm rise, and allow the body's natural recoil to do most of the work. Avoid the temptation to collapse the breath or sigh it out — both patterns signal urgency. Steady and quiet is what you are aiming for.

The Hold (Count of Four — Post-Exhale)

Holding after the exhale — with the lungs near empty — is the least comfortable phase for most people, especially under stress, because the rising carbon dioxide levels in the blood create an urge to breathe. That urge is not danger; it is chemistry. Tolerating it for even a short count is a training in itself, because it teaches the nervous system that urgency can be noticed without being obeyed. This phase also allows the diaphragm to rest in its naturally elevated position before the next cycle, which primes it for a strong, full inhalation.

When You Cannot Box: Simpler Protocols for Chaos

The full four-count box is excellent in the seconds before contact — when you sense trouble coming and have a moment to prepare — and in the immediate aftermath, when the confrontation has passed and you need to bring yourself back down. 

But in the middle of a physical struggle, counting to four in your head is often a fantasy. The brain is simply too loaded.

For those moments, the goal simplifies to one thing: keep breathing. The most common acute error under extreme stress is breath-holding. People clench their jaw, brace their core, and forget to breathe entirely. If you do nothing else, training your body to exhale on effort — exhale when you strike, exhale when you grapple, exhale when you push — keeps the respiratory cycle moving and prevents the dangerous pressure buildup and CO2 accumulation that accelerates fatigue and cognitive shutdown.

A simpler mid-conflict protocol is the physiological sigh, a naturally occurring respiratory pattern the brain uses to reset the lungs. 

It is a double inhale through the nose — breathe in, then sniff in a little more to top off — followed by a long slow exhale through the mouth. 

Researchers at Stanford, including Andrew Huberman and David Spiegel, have documented that even a single physiological sigh is more effective at rapidly reducing acute physiological arousal than several cycles of regular slow breathing (Balban et al., 2023). It takes about five seconds and works even in high-noise, high-chaos environments because it requires no counting, no rhythm, just one distinctive breath pattern.

Training the Breath Before the Storm Arrives

None of this works if you only try it the first time when someone is trying to hurt you. The vagal response — the body's ability to come back from sympathetic overdrive — is trainable, but it is trained in the dojo, in the gym, and in the quiet of your own practice, not improvised in a parking lot. What you are building through deliberate practice is what sports scientists call heart rate variability (HRV), which is essentially the flexibility of your autonomic nervous system to shift gears rapidly. High HRV is associated with better performance under stress across domains from elite athletics to combat medicine (McCraty & Shaffer, 2015).

In Isshin-ryū practice, as in most classical Okinawan karate-jutsu lineages, the breath has never been ornamental. The kiai — the sharp exhalation accompanying a committed technique — is not a shout of bravado. It is diaphragmatic breath weaponized: it stabilizes the core, engages the musculature through chinkuchi, and simultaneously forces the exhale phase that keeps the respiratory cycle from stalling. Every kata contains embedded breathing instruction. Practicing those patterns under increasing physical and psychological load is what wires the pattern into the procedural memory — the part of the brain that keeps working when the adrenaline has washed out the frontal lobes.

When you train box breathing during aerobic stress — during sparring, during heavy bag rounds, during hard conditioning sets where your heart rate is already elevated — you are teaching the nervous system that it is possible to execute this protocol under load. You are, in effect, giving the sympathetic system a rehearsed exit ramp. The goal is automaticity: that when things go sideways, the breath becomes the anchor that brings everything else back into function without requiring conscious deliberation.

Carbon Dioxide, Oxygen, and the Hyperventilation Trap

There is a physiological wrinkle worth understanding, because it is counterintuitive. The urge to breathe is not actually driven primarily by oxygen levels. It is driven by carbon dioxide levels in the blood. Rapid, shallow chest breathing — exactly the pattern stress produces — blows off CO2 faster than the body generates it. This is hyperventilation, and it creates a cascade of its own: the blood becomes more alkaline, blood vessels — including those in the brain — constrict, and paradoxically, less oxygen is delivered to the tissues even though you are breathing faster. Muscles begin to cramp. Tingling appears in the hands and face. Anxiety increases. The body interprets all of this as more threat, which keeps the sympathetic system firing (Ley, 1988).

Slow, diaphragmatic breathing with a slightly extended exhale corrects this by allowing CO2 to rebuild to its normal range, which dilates the cerebral blood vessels, improves oxygen delivery, and takes the false-alarm signal off the table. This is why the counting matters: it is not ritual, it is physiological engineering. You are deliberately slowing yourself below the hyperventilation threshold so the chemistry can stabilize.

Putting It Together: A Practical Sequence

Here is how these elements come together in a realistic sequence:

Before (Threat Recognition Phase): The moment your threat recognition circuitry fires, begin box breathing if you have three seconds or more. Nasal in for four, hold for four, out for four, hold for four. Even one cycle does measurable work.

During (Active Conflict Phase): Forget counting. Focus on exhaling with effort. Strike on the exhale. Move on the exhale. If the situation allows a half-second reset, use a physiological sigh — double inhale through the nose, long exhale through the mouth. Do not hold your breath.

After (Recovery Phase): Return immediately to box breathing or slow diaphragmatic breathing. Two to four cycles will begin to bring heart rate down. Assess, communicate, breathe. The cortisol takes longer to clear — sometimes hours — but the acute epinephrine spike can be managed in under two minutes of deliberate breathing.

The Quiet Authority of the Breath

The adrenal response is not your enemy. It is the oldest survival machinery in the animal kingdom, and it wants you to live. But it was designed for a world where threats lasted seconds and the correct response was almost always run or bite. Modern conflict, particularly the kind involving another human being with full decision-making capacity, requires you to retain far more cognitive function than pure sympathetic activation allows. The breath is the lever between those two states — between the flooded, tunnel-visioned, fine-motor-compromised version of yourself and the version that can still make the right call.

Train it. Practice it under load. Know which phase of the breath does which job. And understand that this is not a soft skill borrowed from the yoga studio — it is a hard physiological tool that the people most serious about surviving violent conflict have relied upon, studied, and refined for as long as human beings have had the wit to notice that their breathing changed when danger came.

References

Balban, M. Y., Neri, E., Kogon, M. M., Weed, L., Nouriani, B., Jo, B., Holl, G., Zeitzer, J. M., Spiegel, D., & Huberman, A. D. (2023). Brief structured respiration practices enhance mood and reduce physiological arousal. Cell Reports Medicine, 4(1), 100895. https://doi.org/10.1016/j.xcrm.2022.100895

Grossman, D., & Christensen, L. W. (2008). On combat: The psychology and physiology of deadly conflict in war and in peace (3rd ed.). Warrior Science Publications.

Lehrer, P. M., Vaschillo, E., & Vaschillo, B. (2000). Resonant frequency biofeedback training to increase cardiac variability: Rationale and manual for training. Applied Psychophysiology and Biofeedback, 25(3), 177–191. https://doi.org/10.1023/A:1009554825745

Ley, R. (1988). Panic attacks during sleep: A hyperventilation-probability model. Journal of Behavior Therapy and Experimental Psychiatry, 19(3), 181–192. https://doi.org/10.1016/0005-7916(88)90028-1

McCraty, R., & Shaffer, F. (2015). Heart rate variability: New perspectives on physiological mechanisms, assessment of self-regulatory capacity, and health risk. Global Advances in Health and Medicine, 4(1), 46–61. https://doi.org/10.7453/gahmj.2014.073

Porges, S. W. (2011). The polyvagal theory: Neurophysiological foundations of emotions, attachment, communication, and self-regulation. W. W. Norton & Company.

Siddle, B. K. (1995). Sharpening the warrior's edge: The psychology and science of training. PPCT Research Publications.

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