At the end of the last post in this series — the Mindset pillar — I teased what was coming next.

Breathing. Specifically, why the way you breathe right now — most likely through your mouth, shallowly, chaotically — is undermining your performance, your sleep, and your stress response.

That was not hyperbole.

You take roughly 20,000 breaths a day. It is the only autonomic function you can also consciously control — the hinge point between your voluntary and involuntary nervous systems. And for most people reading this, the vast majority of those 20,000 breaths are making things worse. Not by a little. By a lot.

Let’s talk about why, and what to do about it.

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James Nestor changed how I think about this

I referenced James Nestor in the walking post — noting that what he did for breathing, Conley and McDowell did for walking. Let me back up and actually give him his due here, because his book Breath: The New Science of a Lost Art is the text that cracked this topic open for me and for a lot of people.

Nestor spent years reporting on breathing, including blocking one nostril and spending ten days breathing exclusively through his mouth while scientists monitored everything that happened. Blood pressure went up. Snoring started. Sleep deteriorated. Cognitive function declined. Athletic performance dropped.

Then he switched to nasal-only breathing for ten days. Everything reversed.

His core finding: humans a long time ago breathed correctly. We have systematically unlearned how. The fossil record shows it — ancient skulls have wide jaws, spacious airways, straight teeth. Modern skulls, starting roughly with the industrial revolution and accelerating sharply with the rise of processed food and sedentary indoor life, show narrowed jaws, crowded teeth, reduced airway diameter, and the structural consequences of a species that has stopped chewing hard food and started breathing through its mouth.

This is not a minor lifestyle quibble. Nestor’s argument is that dysfunctional breathing is at the root of conditions ranging from anxiety and poor sleep to cardiovascular disease and immune dysfunction. And the fix — at its most fundamental — is embarrassingly simple: close your mouth.

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Patrick McKeown on The Ready State

If Nestor is the journalist who broke the story, Patrick McKeown is the scientist who built the clinical framework around it.

McKeown is the founder of the Buteyko Clinic, author of The Oxygen Advantage, and one of the world’s most rigorous researchers on functional breathing. He recently appeared on The Ready State podcast with Kelly and Juliet Starrett — and given that Kelly Starrett is the person I credit for transforming how I think about mobility, movement, and physical preparation, this was a conversation I paid close attention to.

The key ideas from that episode:

Most people are chronically over-breathing. This sounds counterintuitive — we tend to think more breath equals more oxygen. But breathing too much actually depletes CO2, which is not just a waste gas. CO2 is the signal that tells your red blood cells to release oxygen to your tissues. When you over-breathe and drive down CO2, your body actually delivers less oxygen to where it needs to go, despite you breathing more. Paradox. A real one.

The BOLT score is your breathing baseline. BOLT stands for Body Oxygen Level Test. Take a normal breath in through your nose, exhale normally, then pinch your nose and time how long it takes before you feel the first urge to breathe. Not the maximum you can hold — just the first urge. A score under 20 seconds indicates dysfunctional breathing. Elite endurance athletes typically score 40 seconds or more. Most sedentary adults score 10 to 15 seconds. This one number tells you a lot about your CO2 tolerance, your anxiety baseline, and your recovery capacity.

McKeown also covers why women experience breathing and sleep differently than men — particularly around menopause — and why many sleep disorders in women may be going undiagnosed because sleep studies were historically designed around male breathing patterns. Example number 5,345 of how women are not just small men.

Mouth breathing in kids is a hidden developmental crisis. Mouth breathing in children has been proven to cause poor academic performance, craniofacial deformities, dental problems, and behavioral issues. If you have kids, this episode is worth paying very close attention to.

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How to Measure Your CO2 Tolerance: The CO2 Exhale Test

Jesse Coomer’s The Language of Breath offers a complementary test to the BOLT score — one I’ve found particularly useful because it measures a different dimension of CO2 tolerance. This protocol was originally developed by elite fitness trainer Brian Mackenzie. It’s the one with which Foghorn athletes are the most familiar.

Where the BOLT score measures how long you can go before the urge to breathe strikes, the CO2 Exhale Test measures how long and slowly you can exhale — which reflects your nervous system’s relationship with CO2 and your actual breathing control.

Wanna give it a shot right now? Before taking the test, breathe normally. Don’t practice or mimic the test first; just breathe naturally. You will need a stopwatch.

Step 1: Take a deep breath through your nose and relax it out to neutral lungs.

Step 2: Wait until you feel the urge to breathe, then take another deep breath and relax it out to neutral lungs.

Step 3: Wait until you feel the urge to breathe again, then take a deep and full breath through your nose.

Step 4: Begin the timer as you exhale through your nose for as long and as slowly as you can.

Stop the stopwatch when any of the following occur: you completely run out of air; you swallow; or there is any interruption in the airflow as you exhale.

Your time is your score.

Scores and their meaning:

0 to 20 seconds: Very poor CO2 tolerance. Retest the following day to confirm — many people score low when under significant stress or illness. Before working on CO2 tolerance techniques, get this score to at least 21. Cardiovascular exercise is one of the best ways to improve it naturally. Do not jump into advanced CO2 tolerance training at this stage; it will add stress rather than improve tolerance.

21 to 40 seconds: Average modern human. You can begin CO2 tolerance training. You have significant room for improvement. This is where most of my athletes start out.

41 to 60 seconds: Advanced. A healthy range. You generally have a relaxed and smooth breathing pattern and can slow your breathing down relatively easily after exertion.

61 to 80 seconds: Elite. Your natural breathing pattern is slow and smooth, you rarely breathe more than you need to, and you experience increased resilience against stressors.

80 seconds and above: Master. You have become very comfortable with CO2 and are highly resilient against stress. I have athletes who’ve been training with me for a few years—which means they’ve been training their breath with me for that long—who are bumping along towards two and a half minutes. Yep.

Take both tests — the BOLT and the CO2 Exhale Test — and you have a meaningful baseline picture of your breathing health. Most people are surprised by how low they score. That’s not a reason for discouragement. It’s a reason to train.

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The Language of Breath: where conscious meets unconscious

I recently read Coomer’s The Language of Breath — endorsed by McKeown himself , by the way — and it has a framing I really like.

Coomer’s core argument is that breathing is the one function where the conscious and unconscious self meet. Your heart beats without you. Your digestion runs without you. Your immune system operates without you. But your breath does all of this automatically — and also responds instantly to your conscious direction. It’s the bridge.

He cites the research: 95 percent of brain activity is unconscious. Your senses are reporting approximately eleven million pieces of information at any given moment. You consciously process about forty. The rest — your heart rate, your posture, your emotional state, your stress response — is being handled by your unconscious self, constantly, relentlessly, using everything it knows about your history and your environment.

Your breath is continuously informing that system. When you mouth-breathe, shallowly, rapidly, your unconscious self interprets this as a threat signal. It doesn’t know you’re just sitting at a desk answering email. It knows your breathing looks like the pattern of someone in danger. And it responds accordingly.

Coomer’s framing: “Most of our troubles in the modern age come from a lack of awareness of these foundational relationships.” We have become, in his words, mind-body machine operators rather than whole beings. Breathing, done correctly, is how you start to close that gap.

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Jaws: the structural problem underneath everything

No conversation about dysfunctional breathing is complete without talking about the jaw.

Sandra Kahn and Paul Ehrlich open the relevant chapter of Jaws: The Story of a Hidden Epidemic with a line that I love: “Our jaws are built for a Stone Age diet, but we’re living in a Big Mac environment.”

Sheee-it. Nailed it.

That single sentence explains a remarkable amount. Our jaws are built for a Stone Age diet — tough meats, fibrous tubers, nuts, fruits that required real chewing effort, hours of it daily. What we get instead is soft bread, yogurt, applesauce, processed food bolted down in minutes. Kahn and Ehrlich identify three core areas of cultural shift that have driven the modern jaw crisis: what we eat and how we eat it, slack jaw and prevailing oral posture, and indoor living and how we breathe. All three are interconnected. All three are getting worse.

The structural consequences are measurable and serious. The human jaw has been shrinking for roughly 300 years. The result is narrow palates, crowded teeth, reduced airway diameter, and a structural predisposition to mouth breathing, snoring, and sleep-disordered breathing. Children of parents who cut food into tiny pieces — out of fear of choking — miss out on the learning experience of how the tongue and muscles coordinate to maximize extraction of nutrients from a meal, and don’t get the muscle exercise needed for healthy jaw development.

Orthodontia fixes the cosmetic symptoms. It doesn’t fix the airway.

If you struggle to breathe through your nose, there may be a structural reason. Addressing it — through myofunctional therapy, palate expansion, or airway-focused dental work — may matter more than any breathing protocol. And chewing more — harder foods, longer duration, starting in childhood — is the lowest-cost, highest-impact intervention available.

In the meantime, work on proper oral posture—mouth closed, teeth lightly touching, tongue pressed up against the soft palate. Simple, right? Kids, too.

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What the research is showing

The science on breathwork has matured significantly in recent years.

A meta-analysis of randomized controlled trials showed that breathwork interventions significantly reduced self-reported stress, anxiety, and depressive symptoms. Slow-paced breathing was particularly effective in promoting parasympathetic activity and increasing heart rate variability.

Even single sessions of as little as two minutes of slow breathing have been shown to increase HRV, suggesting breathwork is accessible, cost-effective, and immediately impactful.

A 2024 randomized controlled trial published in JAMA Open Network found that physicians who regularly practiced structured breathwork over two months experienced significant decreases in insomnia, stress, and anxiety.

For athletes, the data on nasal breathing during training is accumulating. McKeown’s Oxygen Advantage framework — training with nasal-only breathing during moderate-intensity work — has been shown to improve CO2 tolerance, increase VO2 efficiency, and reduce the ventilatory threshold. Kipchoge ran his sub-two-hour marathon with his mouth closed for most of it! Worth noting.

If you are using Whoop and your HRV is suppressed, your breathing habits during the day and during sleep may be a primary driver.

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Practical takeaways

The good news: the interventions are free, require no equipment, and can begin immediately.

Take both tests. Do the BOLT score and the CO2 Exhale Test today. Write both numbers down. These are your baselines. Return to them in four weeks of nasal breathing practice and see what changes.

Tape your mouth at night. McKeown and Nestor both do it. Micropore medical tape across the lips encourages nasal breathing during sleep, reduces snoring, and improves sleep quality for many people within a few nights. Works for me.

Train nasal. During easy and moderate-intensity training — your Zone 2 work, your warm-up, your cool-down — breathe exclusively through your nose. If you can’t maintain nasal breathing at a given pace, slow down. Your pace will catch up to your breathing capacity over weeks of training.

Use the downregulation protocol before bed. Nasal breathing only, belly-led inhales, exhale twice as long as the inhale, 4-count in and 8-count out, three to five minutes minimum, thirty minutes before bed. This is your direct dial into the parasympathetic nervous system. Use it every night.

Practice box breathing for stress. Four counts in through the nose, four counts hold, four counts out through the nose, four counts hold. Four rounds. This is what military units use for acute stress regulation.

Chew. Hard foods, more often. Sugar-free xylitol gum. Tougher proteins. Raw vegetables. Your jaw is a use-it-or-lose-it structure — and relatively little chewing early in life can alter the development of your entire face, jaws, and airways.

Learn the Wim Hof Method for upregulation. And here’s one more big one, as every Foghorn athlete knows: Wim Hof Method upregulation breathing. Every training session I run – bootcamp, 1:1, Zoom – begins with a round of Wim Hof Method breathing. This is the other side of the breathwork coin. Where the downregulation protocol dials you down, WHM dials you up. It is a deliberate, controlled form of hyperventilation that primes your nervous system, floods your body with adrenaline, alkalizes your blood, and generates a state of heightened focus and physical readiness before you start moving. I’ve been using it as a warmup tool for years, and the science has been catching up fast. A 2024 systematic review in PLOS ONE found promising evidence for WHM’s effects on inflammatory response and exercise performance. A randomized controlled trial published in 2024 showed WHM practice reduced depressive symptoms and stress – with high-arousal WHM-style breathing specifically outperforming calming protocols for some individuals, challenging the assumption that breathwork should always be downregulatory. A 2023 study in Scientific Reports found that 15 days of WHM practice improved blood pressure, HRV, and subjective vitality.

The protocol: sit or lie down comfortably. Take 20 to 30 deep, powerful breaths – inhale fully through the nose or mouth, then let go without forcing the exhale. Keep a steady, rhythmic pace. After the final exhale, hold your breath with empty lungs for as long as comfortable – most people start at 60 seconds and build to two or three minutes over time. Then take one deep recovery breath, hold it for 10 to 15 seconds, and release. That’s one round. Do three to four rounds. You will feel tingling, lightheadedness, and a warmth spreading through your body – all normal, all physiological. Do not do this in water, while driving, or anywhere a loss of consciousness would be dangerous. When you finish, you are primed. Now go train. For the full method, Wim Hof’s book is the place to start: The Wim Hof Method.

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Next up in this series: Nutrition. Specifically, why the most important dietary intervention most people could make costs nothing, requires no supplements, and is not a diet at all.

All ten pillars of the Slowfit Method® live here on Substack and at foghornfitness.com. Paid subscribers get deeper dives, workout programming, and the full resource library.

The Slowfit Method® app puts Coach Keir AI in your pocket — trained on all ten pillars including this one. Ask it anything about your breathing, your CO2 tolerance, your sleep, or your stress. One hundred people have already downloaded this app, and I’ve got nine 5-star views on the App Store. Check it out!

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And if you’re interested in a 1:1 private coaching relationship, you can book sessions at foghornfitness.com . You can even book a free intro discussion with me to learn more about how the Slowfit Method® could amp up your performance, health and happiness.

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Stick around.

Best,

Keir

Keir J. Beadling is a human performance coach, founder of Foghorn Fitness, and creator of the Slowfit Method®. He coaches athletes and everyday humans in San Francisco and worldwide. More information is available at foghornfitness.com.