For decades, people have described breathwork sessions in language that sounds a lot like a psychedelic trip: dissolving boundaries, vivid imagery, a sense of unity, waves of emotion moving through the body. A study published in Nature Portfolio last year puts numbers behind that resemblance, and it does something more interesting than simply confirming an anecdote — it identifies the physiological switch that seems to flip an ordinary breathing exercise into a genuinely altered state.

Researchers at the MIND Foundation in Berlin recruited 61 experienced practitioners and tracked them through full sessions of two of the most widely practiced circular breathwork styles, Holotropic and Conscious-Connected breathwork. Every 15 to 30 minutes, participants exhaled into a portable capnography device that measured their end-tidal CO2 (etCO2) — a direct readout of how hard their bodies were working to blow off carbon dioxide — and rated the depth of their experience on a simple five-point hand-sign scale. A subset of participants also breathed normally throughout the same sessions, giving the researchers a built-in control group exposed to identical music, group energy and facilitation, but without the deliberate hyperventilation.

What happens to CO2 during circular breathing

The pattern was unambiguous. Active breathers drove their average etCO2 down from a resting baseline of around 37 mmHg to roughly 20 mmHg, and some individuals bottomed out as low as 10–15 mmHg — levels that would flag a medical emergency in almost any other context. Passive breathers, sitting through the same session but breathing normally, barely moved off baseline. This is the same physiological territory mapped out in research on the Wim Hof Method, where structured hyperventilation followed by breath retention has repeatedly been shown to lower CO2 saturation, shift blood pH toward alkalinity, and trigger vasoconstriction — effects that are usually framed around the Wim Hof Method’s cold exposure work, but that clearly extend to circular breathwork more broadly.

Cerebral blood vessels are particularly sensitive to this shift: falling CO2 constricts them, and that constriction — rather than anything mystical about the breathing pattern itself — appears to be doing a lot of the work.

Tracking altered states in real time

The real value of this study is that it didn’t just ask people to recall their experience afterward — it captured how deep the state ran at six separate moments during the session, in step with the CO2 readings. The two measures moved together with striking consistency: the lower a participant’s CO2 dropped, the deeper the altered state they reported, with a correlation of around -0.46 across both breathwork styles. Statistical modelling across all measurement points confirmed that etCO2 predicted experience depth in the active-breath group but not in the passive-breath group, which rules out a simple “it’s just the music and the group” explanation.

There was also a threshold effect worth noting: intense altered states almost never appeared unless etCO2 fell below roughly 35 mmHg, and once it dropped under 20 mmHg, some degree of altered consciousness was nearly guaranteed. Interestingly, once a deep state had been triggered, it tended to persist for a while even as CO2 began climbing back toward baseline in the second half of the session — as if the drop in CO2 acts as an ignition switch rather than something that needs to stay depressed the whole time.

Breathwork versus psychedelics

Participants also completed the two gold-standard questionnaires used in psychedelic research, the Mystical Experiences Questionnaire (MEQ30) and the 11-Dimensional Altered States of Consciousness Scale (11-DASC). Scored against a reference database of clinical psychedelic trials, breathwork produced results that sat close to — and on several subscales statistically indistinguishable from — a standard therapeutic dose of psilocybin, and it scored higher than MDMA on every single subscale of both questionnaires. Scores were somewhat lower than a typical LSD dose, but still far above what would be expected from a placebo condition.

Even the passive breathers, who kept breathing normally throughout the session, registered altered-state scores meaningfully above placebo — a reminder that music, group sharing and facilitated support carry real weight of their own. But the active breathers went much further, and the size of that gap tracked closely with how far their CO2 had fallen.

What it meant a week later

A subset of participants completed standard wellbeing and mood questionnaires one week before and one week after their session. On average, scores on both measures improved, and — notably — the size of that improvement was predicted by how deep the altered state had been during the session itself, not simply by having attended a breathwork session. Saliva samples told a more layered story: markers of sympathetic nervous system activity dropped after the session, while an inflammatory marker rose in the short term, a pattern the researchers interpret as consistent with a temporary physiological challenge followed by a shift toward relaxation — the kind of hormetic stress response also described in cold exposure research.

It’s worth being precise about what this data does and doesn’t show. This is a single exploratory study with modest sample sizes, self-selected experienced practitioners who likely expected the practice to help them, and a control group too small to draw firm conclusions from. The researchers themselves flag all of this. What the study does establish convincingly is the physiological mechanism: a measurable, dose-dependent relationship between how far CO2 drops and how altered the resulting state is.

A shared mechanism across very different practices

One of the more elegant threads running through the paper is the idea of “pivotal mental states” — a framework proposing that a range of physiological disruptions, from breath-driven hyperventilation to cold exposure, fasting or sensory overload, can push the nervous system into a state of heightened plasticity and altered perception through a common serotonergic pathway. Psychedelics access that same switch pharmacologically; breathwork and cold immersion access it by physically perturbing the body’s normal equilibrium. It’s a useful reminder that the three pillars at the heart of the Wim Hof Method — breathing, cold, and focus — are not three unrelated tools, but three different doors into what may be the same underlying physiological process.

What made Holotropic and Conscious-Connected breathwork particularly interesting to compare here is how similar their effects turned out to be, despite Holotropic sessions running roughly twice as long. Longer didn’t mean deeper. The CO2 trajectory and the experience-depth trajectory both peaked early and tracked almost identically between the two formats — suggesting that it’s the intensity of the breathing pattern, not the duration of the ritual around it, that does most of the work