“This feels like suffocation.”
That was my first thought after completing a few sets of swings a couple of years ago during a training session in which I had decided to nasal breathe the entire time. While I held out for a bit, I ultimately had to briefly switch to mouth breathing before continuing on.
In contrast, my mouth stayed closed throughout my entire training session the Sunday before last, including during the latter half of a 10-minute clean and jerk section in which my heart rate stayed over 160 beats per minute. I still wanted to switch to mouth breathing a couple of times, but the urge was nowhere near what it had been during that first session roughly two years ago.
Obviously, something has changed—but it’s not (just) willpower.
Why it feels like suffocation (and what’s actually happening)
There’s a widespread (and understandable) assumption that we need more oxygen when we’re working hard and breathing feels insufficient.
But that’s not exactly what’s happening.
The primary trigger for the urge to breathe isn’t usually falling oxygen (O₂) levels. It’s rising carbon dioxide (CO₂). Chemoreceptors (sensory receptors that detect chemical changes) in our brainstems and at the fork of our carotid arteries monitor CO₂ concentration in the blood, and when it climbs past a certain threshold, they send an alarm: breathe.
Oxygen levels have to fall quite substantially before they send the same signal. Under most normal training conditions—even hard ones—our O₂ supply is fine. The suffocation feeling is our CO₂ sensitivity doing its job, loudly.
This distinction matters because CO₂ sensitivity is a lever that almost nothing in a typical fitness program pulls. And it is trainable—we just don’t do it.
But why should we?
Because improving it also affects a secondary mechanism. Carbon dioxide plays a role in how efficiently O₂ gets delivered to working muscles: in the presence of more CO₂, hemoglobin (the protein responsible for transporting O₂ and CO₂ in the body) releases oxygen more readily. This is called the Bohr effect, and it means that some extra CO₂ in the blood isn’t just tolerable—it’s actually useful.
What changes, and what doesn’t
Patrick McKeown, author of The Oxygen Advantage, argues that most people—particularly those who habitually breathe through their mouths or over-breathe at rest—have chronically low CO₂ tolerance. From a training perspective, this means that when intensity ramps up and CO₂ accumulates quickly, the “breathe alarm” fires at a threshold that is, in a sense, too sensitive.
What seems to actually happen with practice comes in two speeds. The fast part: your tolerance for that alarm—for the sensation of rising CO₂—improves within weeks, fairly reliably. That’s not the alarm’s threshold moving; it’s you getting less rattled by the signal. The slower part: an actual shift in where the threshold sits looks real too, but the evidence for it comes from years of sustained practice, not weeks. And in fairness, that evidence is about breath-hold training and years of general aerobic conditioning—nobody has directly tested nasal breathing, specifically, as the thing doing the recalibrating. Nasal breathing seems to be a reasonable way to apply a similar stimulus—mild, repeated CO₂ elevation during exercise—but I can’t claim it’s the proven lever.
Still, that two-speed pattern matches my own experience: after more than two years of regularly practicing nasal breathing during everyday life and training—including kettlebell training, rucking, climbing, and even BJJ—I can now almost exclusively do it through most training sessions and at fairly high heart rates. (As I mentioned above, the urge to breathe through my mouth still occurs. It’s just more tolerable than it was before, and I don’t need to give in to it as often.) Two years tracks a lot closer to “slow and real” than “fast and easy.”
Now I want to be precise about what I’m saying here. Nasal breathing during training almost certainly doesn’t improve performance in the direct sense of faster times or heavier lifts compared to mouth breathing at equal effort. However, based on McKeown’s claims and my own experience, the adaptations that seem to occur—better tolerance for that rising-CO₂ sensation, possibly more efficient O₂ use, or both—cause us to feel more comfortable at any given level of effort.
That comfort doesn’t make any single rep or interval faster—but it does mean we’re less likely to pause to catch our breath when we don’t actually need to. Less unnecessary rest adds up to more total work across a session, and more work, over time, tends to produce better results.
The recovery side of nasal breathing
Our nervous systems have two dominant operating modes: sympathetic (fight-or-flight, elevated heart rate, stress hormones active) and parasympathetic (rest-and-digest, heart rate down, recovery possible). After hard training sessions or stressful events, the goal for improving recovery is to shift from the first mode to the second as efficiently as possible. And the way you breathe—specifically, slow nasal exhalation—is one of the most direct levers you can pull to make that shift occur.
Extended exhalation—in general, although this is usually easier via nasal breathing than mouth breathing—activates the vagus nerve, which drives parasympathetic response. Nasal breathing adds another mechanism: the nasal passages and sinuses produce nitric oxide, which acts as a vasodilator and supports oxygen delivery.
The best part is that you don’t need an elaborate protocol to take advantage of either effect. A few minutes of deliberate breathing after training—slow nasal inhalation, longer nasal exhalation—can meaningfully accelerate the transition out of a high-sympathetic state.
Of course, if you prefer something slightly more structured, Andrew Huberman has popularized a technique called the physiological sigh—developed from research by Jack Feldman’s lab at UCLA—which has been shown to be particularly effective at reducing acute physiological and psychological stress. The technique: a double inhalation through the nose (a full inhalation, followed immediately by a short second inhalation to fully expand the lungs), then a slow, complete exhalation through the mouth. One or two of these can reset the system faster than normal breathing.
Putting it into practice
If you’re already training regularly and want to experiment with nasal breathing during exercise:
Start with warm-ups and lower-intensity work only. The discomfort will be real—that’s your CO₂ alarm doing exactly what it’s designed to do, before your tolerance for the sensation has had a chance to build. Resist the urge to interpret “this feels hard” as “this isn’t working,” and give it several weeks before extending into higher-intensity training.
If nasal breathing during training isn’t where you are yet (or isn’t of interest):
After your next hard session, spend two or three minutes breathing through your nose—no counting required. Just slow down the inhalation and make the exhalation a little longer than the inhalation. Then notice whether your heart rate and sense of urgency settle faster than they normally do.
The first is a longer-term project. The second you can try today.
† If you have a cardiovascular condition or any breathing-related health concern, check with your doctor before significantly changing how you breathe during training.