Breathing, explained

Why train your breathing?

Regular breath training can increase tolerance to carbon dioxide, contribute to calmer everyday breathing, and lower perceived exertion during physical activity. Many techniques originate from endurance and apnea sports, but are now also used for general wellbeing.

How breathing affects stress — the mechanisms behind it

Breathing is unusual in the body: it happens automatically, yet it can also be controlled consciously. This makes it a direct interface to the autonomic (vegetative) nervous system, which otherwise operates largely outside our voluntary control. This system has two parts: the sympathetic part, which activates the body and gears it toward tension, and the parasympathetic part, which promotes recovery and calm. Under stress the balance shifts toward activation, and breathing typically becomes faster and shallower.

Deliberately slowing your breathing acts precisely here. One key mediator is the vagus nerve. It runs from the brainstem to the heart, lungs and digestive organs and dampens activity there; it is the principal nerve of the parasympathetic (calming) part. Its influence shows up in a well-described phenomenon (respiratory sinus arrhythmia): heart rate speeds up slightly as you breathe in and slows down again as you breathe out. A lengthened exhale strengthens exactly this effect.

One particular breathing rhythm is especially interesting. When the breathing rate is reduced to around six breaths per minute, breathing, blood-pressure and heart-rate fluctuations lock into a timing pattern known as coherence or resonance. In this state the baroreflex also comes into play — a control loop running through pressure sensors in the blood vessels that tunes heart rate and blood pressure to one another. Heart rate variability — the natural fluctuation in the intervals between heartbeats — is considered a measurable indicator of how flexibly the nervous system can switch between activation and calm. Studies show that slow breathing shifts this balance towards calm.

Why regular breathing practice can be supportive

The decisive point is that these relationships can be trained. Just as stress changes your breathing, deliberately calm breathing can help regulate inner tension — and that ability can be built through repetition. Slow breathing and heart-rate-variability training are described as gentle methods that can support the nervous system's self-regulation, the baseline activity of the vagus nerve, and resilience to stress.

Regularity plays an important role: it is not the single deep breath in a stressful moment, but repeated practice that strengthens what research describes as autonomic flexibility — the nervous system's ability to adapt dynamically to internal and external demands. By regularly paying attention to your breath and consciously slowing it down, you train an access route to more easily reach a state of calm and well-being.

Why CO2 matters for the body

Carbon dioxide is often seen as just a waste product of breathing — but in reality it plays a central regulatory role in the body. It isn't a lack of oxygen that triggers the urge to breathe, but primarily the rising CO2 level in the blood: when holding your breath, it's this rise that eventually creates the sensation of needing to breathe again. Carbon dioxide also affects how easily oxygen is released from the blood to the cells (the Bohr effect) — so a certain CO2 level actually improves oxygen delivery to tissue and brain rather than hindering it. Training your CO2 tolerance and gradually raising it teaches you to breathe less and more calmly in everyday life, too. And that has real effects: calmer, more efficient breathing can settle the nervous system, improve oxygen delivery, support concentration, and lower perceived exertion under load.

Overbreathing in everyday life

Overbreathing doesn't only happen in acute stress situations — it often occurs unnoticed in everyday life: many people constantly breathe a bit faster or deeper than the body actually needs, often through the mouth and into the chest rather than the belly. This mild, chronic form of overbreathing also lowers the CO2 level in the blood. And a low CO2 level isn't without consequences: it narrows the blood vessels in the brain and, at the same time, shifts hemoglobin's oxygen-binding curve so that oxygen is released to tissue less readily. The paradox is the same as with acute hyperventilation: even though it seems like you're breathing "more," the cells may actually have less oxygen available. This is exactly where breath training comes in. Learning to breathe more calmly, more slowly, and through the nose gradually raises your CO2 tolerance — and as a result, your everyday breathing automatically becomes more economical. Slower, calmer breathing also activates the calming part of the nervous system (the parasympathetic nervous system) and can help lower stress and inner tension. A healthy relationship with CO2 is therefore far more than a detail for athletes: it's a foundation for calm, relaxed, and efficient breathing.

Which breathing exercises calm you down — and why

Whether breathing has a calming effect depends less on how much you breathe than on how you breathe. The most important lever is the ratio of inhale to exhale: during inhalation, the calming influence of the vagus nerve is briefly dampened and heart rate rises slightly — during exhalation, this influence returns and heart rate drops again. The exhale is therefore the phase that activates the parasympathetic, calming part of the nervous system. Making the exhale longer than the inhale (for example, 4 seconds in, 6 seconds out) deliberately extends this calming phase. Studies show that an exhale that's longer relative to the inhale can increase heart rate variability (HRV) — a sign of increased vagus nerve activity. This is supported by an overall calm breathing pace: at around six breaths per minute, heart rate and breathing fall into a kind of resonance that raises HRV especially strongly. Diaphragmatic breathing is key here — breathing where the movement happens low down, at the diaphragm and in the belly, rather than in the shoulders and chest. Importantly, the goal isn't to inhale a large volume of air, but to breathe calmly, evenly, and economically — because too much air would unnecessarily exhale CO2 and work against the actual goal.

Nose or mouth — which is right?

Two schools of thought face each other in the breathing world. One says: through the nose, consistently, including during practice. The other has you exhale deliberately through the mouth in certain exercises, often against the resistance of almost closed lips — so-called pursed-lip breathing. Both have good reasons, and they contradict each other less than it first sounds.

A lot speaks for the nose. It filters, warms and humidifies the air before it reaches the lungs. Nitric oxide is produced in the paranasal sinuses, travels into the lungs with every breath and widens the blood vessels there. And the narrower path slows the airflow, so breathing becomes slower by itself. In a small study of young adults, nose-only breathing at rest performed measurably better: lower diastolic blood pressure and a greater calming component in heart rate variability. For the breathing that happens around the clock — and during exercise up to a certain intensity — the nose is unquestionably the gold standard.

When exhaling through the mouth comes up, by contrast, it always concerns specific breathing exercises, never everyday breathing. Exhaling through almost closed lips creates back pressure, and that is exactly the point: it lengthens the exhale, makes it more even, and gives you tangible feedback on how fast you are letting the air out. The technique has long been established in respiratory therapy.

The best-controlled study on breathing exercises to date also supports it. Four five-minute exercises were compared over a month, among them box breathing and mindfulness meditation. The best result came from the “physiological sigh”: two short inhales through the nose, then a long exhale through the mouth. It lifted mood the most and lowered resting breathing rate the most — the deciding factor was the emphatically long exhale.

The two findings answer different questions, which is why they do not conflict. The advantage of the nose shows up in continuous breathing: all day, at night, while running. The benefit of pursed lips shows up within an exercise in which the resistance is the active ingredient. A direct comparison of “exhaling through the nose versus through the mouth, in an otherwise identical exercise” does not yet exist. Anyone promising a clear right answer here is going beyond the evidence.

So one does not rule out the other. You can work consistently on nasal breathing in daily life and still do individual exercises with pursed lips — that is not a contradiction but the difference between a default setting and a tool. Conversely, almost every exercise also works with a nasal exhale; it becomes quieter, the exhale often a little shorter, and you have to hold the pace more on your own. Atmea therefore does not commit to one school but takes what belongs to each exercise — and states with every exercise which path is meant.

Sources: Lundberg, The Anatomical Record (nitric oxide from the paranasal sinuses); American Journal of Physiology 2023 (nose versus mouth at rest, n=20); Balban et al., Cell Reports Medicine 2023 (controlled comparison of breathing exercises).

What is the BOLT score?

BOLT stands for Body Oxygen Level Test. After a normal exhale, you time how long it takes until the first noticeable urge to breathe — deliberately not the maximum time you can hold. The score is a proxy for CO2 tolerance and is most meaningful as a trend over several weeks, not as a single reading.

Training CO2 tolerance

Structured training tables with gradually increasing intervals (the CO2 table) are designed to raise CO2 tolerance in a targeted, trackable way, rather than repeating isolated maximal breath holds. The training stimulus does not come from making single holds as long as possible, but from repeating them with ever shorter recovery in between.

Several things happen in the body along the way, and most of them are well studied. The first concerns the urge to breathe itself. That urge does not arise from a lack of oxygen but from rising CO2 in the blood: sensors in the brainstem and on the carotid arteries raise the alarm once a threshold is crossed. In people who train apnea regularly, this ventilatory response to CO2 is measurably weaker. The alarm goes off later and more quietly — at a CO2 level that has not changed.

The second is the involuntary diaphragm contractions. They mark the point where the body stops merely reporting the breath hold and starts actively working against it. With training they set in later.

The third is the diving response, an innate reflex shared by all mammals. As soon as you hold your breath — more strongly if your face meets cold water at the same time — the heartbeat slows and the vessels in the arms and legs constrict. Blood stays where it is needed most: at the heart and brain. With training this response appears sooner and more markedly.

The fourth is the least known: during repeated breath holds the spleen contracts and releases stored red blood cells into the bloodstream. The blood's oxygen carrier rises briefly as a result — one of the reasons why, in a series, the third or fourth attempt often goes better than the first. The effect is temporary and fades after a few minutes.

What does not happen matters just as much. The body does not produce less CO2 — what changes is how you deal with it: when the alarm sounds, how strongly the body reacts, and how well you can tolerate it. Part of the progress is therefore not physical but learned: getting used to a sensation that feels threatening at first and is not.

Sources: research on the diving response and splenic contraction in breath-hold divers (including the Schagatay group, Mid Sweden University); review articles on the ventilatory response to CO2.