Client Resource, Performance
You are probably breathing too much. Overbreathing is common, measurable, and has real consequences for performance, recovery, and sleep. Here is what the evidence shows.
Breathing is the most automatic of physiological functions and the one most people pay the least attention to. It is also, in many cases, done incorrectly in ways that have measurable consequences for performance, recovery, anxiety, and sleep quality. The key misunderstanding is this: the urge to breathe is driven not by falling oxygen levels but by rising carbon dioxide (CO2) levels. CO2 is not merely a waste gas, it is a critical physiological signal that regulates oxygen delivery, blood pH, vasodilation, and bronchodilation.
Overbreathing, breathing at a rate or volume in excess of metabolic demand, lowers arterial CO2 below optimal levels. This produces a counterintuitive effect: despite adequate oxygen in the blood, the Bohr effect means less of it gets released to tissues, because CO2 is required for haemoglobin to release oxygen. The result is a state where you can be simultaneously breathing hard, absorbing plenty of oxygen, and still under-oxygenating working tissues, because CO2 is too low. Building CO2 tolerance is the mechanism for resolving this.
The Bohr effect, described by Christian Bohr in 1904, establishes that haemoglobin releases oxygen more readily in environments of higher CO2 concentration and lower pH. This means that adequate CO2 in tissues is required for efficient oxygen delivery, not just adequate oxygen in blood. Overbreathing, by reducing arterial CO2, directly impairs tissue oxygenation despite adequate lung ventilation.
Most people believe the urge to breathe is driven by falling oxygen (O2) levels. It is not. The primary chemoreceptors that trigger breathing respond to rising CO2 and falling pH in arterial blood, not to oxygen depletion. This is why breath-holding immediately after a deep exhalation is harder than after a breath, you have already expelled CO2 and the trigger fires quickly. It is also why hyperventilation (rapid breathing) produces dizziness, tingling, and sometimes fainting: rapidly blowing off CO2 lowers arterial CO2 (hypocapnia), causing cerebral vasoconstriction and reduced brain perfusion even though blood is fully oxygenated. Understanding this inverts most people's intuition about breathing and exercise.
Practical relevance: People who breathe rapidly and shallowly during moderate-intensity exercise are not necessarily oxygen-deprived. They may be over-ventilating relative to metabolic demand, wasting energy on unnecessary respiratory work and impairing performance through CO2-mediated changes.
Dysfunctional breathing, breathing at a rate or tidal volume above metabolic demand, is significantly more prevalent than clinical practice acknowledges. Estimates vary, but prevalence of dysfunctional breathing patterns in adults with anxiety disorders exceeds 60%, and it is common in otherwise healthy people with occupational stress, poor sleep, or habitual mouth breathing. The consequences include chronic low arterial CO2 (hypocapnia), heightened sympathetic nervous system activation, disrupted sleep, reduced exercise performance ceiling, and a lower CO2 tolerance threshold that makes the urge to breathe feel urgent at intensities where it should be comfortable. This produces a vicious cycle: low CO2 tolerance makes breathing feel harder at a given intensity, which triggers faster breathing, which further lowers CO2.
Self-assessment: The BOLT score (Body Oxygen Level Test) is a simple CO2 tolerance assessment. After a gentle exhalation, measure the time until the first urge to breathe (not maximum breath hold, just first urge). Scores below 20 seconds suggest dysfunctional breathing; above 40 seconds is well-calibrated. Most people score 15–25 seconds initially.
The nose performs functions that the mouth cannot: filtering, humidifying, and warming incoming air; producing nitric oxide (which dilates blood vessels and increases oxygen uptake efficiency in the lungs); and adding resistance to airflow that slows breathing rate and increases CO2 tolerance. Nasal breathing during sleep prevents the hypocapnia, airway drying, and increased arousals associated with mouth breathing, and is associated with better sleep quality, lower apnoea index, and improved overnight recovery. During exercise, nasal breathing is possible at moderate intensities in people with reasonable CO2 tolerance, the point at which mouth breathing becomes necessary during exercise is itself a useful proxy for aerobic threshold, roughly corresponding to zone 2 intensity.
Mouth taping for sleep: Applying a small piece of surgical tape or a purpose-made device to close the mouth during sleep is a low-risk intervention for those who mouth-breathe at night. Strype nasal strips worn during sleep can complement this by improving nasal airflow. These are adjunct tools, not substitutes for addressing underlying nasal obstruction or sleep apnoea.
CO2 tolerance, the threshold at which rising CO2 triggers the urge to breathe, adapts in response to deliberate training. Practices that increase CO2 tolerance include reduced breathing exercises (consciously breathing less than you feel you need to for short periods), breath-hold walking, and extended nasal breathing practice at rest and during low-intensity exercise. The Buteyko method, the most studied structured approach to CO2 tolerance training, has RCT evidence for reducing asthma symptoms and medication use, and observational evidence for improving exercise performance and reducing anxiety. Patrick McKeown and others have translated this into structured protocols accessible to non-clinical populations.
Basic CO2 tolerance training protocol: 5 minutes of reduced breathing practice daily, breathing through the nose with reduced tidal volume until a mild air hunger is felt, sustained for 3–5 minute periods. This is distinct from breath-hold training (not maximal hold, just reduced volume). Progress by monitoring BOLT score weekly.
An undertrained CO2 response means the urge to breathe becomes urgent at lower exercise intensities, increasing perceived effort, accelerating breathing rate, and increasing the energy cost of respiration (breathing muscles consume oxygen too). In highly trained endurance athletes, the ventilatory threshold, the point at which breathing rate accelerates disproportionately to workload, occurs at higher absolute workloads, in part because of greater CO2 tolerance. Improving CO2 tolerance through breathing practice and consistent zone 2 training (which habituates the chemoreceptors to higher CO2 levels during sustained activity) raises the ventilatory threshold and reduces perceived effort at a given exercise intensity.
NTCZ relevance: The Engine pillar of The Benchmark assesses cardiorespiratory function including ventilatory response. Identifying individuals with an abnormally early ventilatory threshold provides diagnostic information about breathing pattern as well as aerobic fitness.
Anxiety activates the sympathetic nervous system, increasing breathing rate and decreasing CO2. Low CO2 activates the sympathetic nervous system through chemoreceptor signalling and produces physical symptoms (tingling, lightheadedness, chest tightness, increased heart rate) that are experienced as anxiety symptoms, completing a self-reinforcing loop. This mechanism means that addressing breathing patterns is a legitimate component of anxiety management, not as a replacement for psychological intervention, but as a direct physiological interruption of the sympathetic activation cycle. The physiological sigh, a double inhale through the nose followed by a long slow exhale, is the most evidence-backed single breath pattern for rapid parasympathetic activation and CO2 balance restoration.
The physiological sigh: Research by Huberman Lab (Stanford) and predecessors shows that the physiological sigh, double inhale, extended exhale, is the fastest single breathing technique for reducing acute physiological stress. One to three repetitions produce measurable HRV improvement within 30 seconds.
Sleep is the longest period of sustained breathing in the day, and the quality of breathing during sleep directly affects sleep architecture, recovery quality, and morning HRV. Mouth breathing during sleep produces hypocapnia, reduces nitric oxide production, dries the upper airway (increasing snoring and apnoea events), and activates sympathetic tone in a way that fragments sleep. Observational data from sleep studies consistently shows that nasal breathing is associated with more slow-wave and REM sleep, lower apnoea-hypopnoea index, and better morning cortisol profiles. For people who are already using wearables to track sleep and HRV, trialling nasal breathing techniques or mouth taping (in the absence of nasal obstruction) often produces measurable improvements in overnight HRV and sleep quality within days.
Practical starting point: If you wake with a dry mouth, experience significant snoring, or your wearable shows consistently fragmented sleep, mouth breathing during sleep is worth investigating. Nasal strips to improve airflow plus a trial of soft mouth tape (not rigid, Somnifix or similar) addresses this at low cost and low risk.
The counterintuitive insight in the breathing research is that better breathing typically means breathing less, not more. Overbreathing is common, impairs performance, disrupts sleep, exacerbates anxiety, and is almost never identified in standard health assessments because it does not show up in resting blood tests or routine spirometry.
The practical interventions are straightforward: nasal breathing during rest and low-intensity exercise, CO2 tolerance training through reduced breathing practice, nasal breathing during sleep, and consistent zone 2 training which trains the chemoreceptors to tolerate higher CO2 levels. These are low-cost, low-risk, and often produce measurable improvements in HRV, sleep quality, and exercise performance within weeks.
Book The Benchmark →Dempsey JA et al., Consequences of exercise-induced respiratory muscle work. Respir Physiol Neurobiol 2006;151(2–3):242–250. Ventilatory work and exercise performance.
McKeown P, The Oxygen Advantage. William Morrow 2015. The Buteyko-derived CO2 tolerance framework; extensively cites the peer-reviewed literature on breathing physiology.
Courtney R, The functions of breathing and its dysfunctions and their relationship to breathing therapy. Int J Osteopath Med 2009;12(3):78–85. Dysfunctional breathing prevalence and mechanisms.
Kox M et al., Voluntary activation of the sympathetic nervous system and attenuation of the innate immune response in humans. PNAS 2014;111(20):7379–7384. Breathing practice and autonomic modulation.
Bohr C, Hasselbalch K, Krogh A, Concerning a biologically important relationship, the influence of the CO2 concentration of the blood on its oxygen binding. Skand Arch Physiol 1904;16:402–412. The original Bohr effect paper, foundational to all CO2/O2 delivery physiology.