Physiological Coherence: What It Is and What It Isn't

What is physiological coherence?

Physiological coherence is a pattern in the heart rhythm. Heart rate always rises and falls a little from beat to beat. In a coherent state those rises and falls line up into one smooth, almost sine-wave-like oscillation with a single dominant slow frequency, instead of a jumble of faster and slower changes [S1]. The pattern appears most clearly when you breathe slowly, close to your resonance rate, which for most adults is about 5.5–6 breaths per minute (around 0.1 Hz).

The term matters for ONDA because coherence is one of the signals the app shows during a practice. This page sets out what the term measures, how it differs from heart rate variability, and which popular claims about it are not supported.

How is it measured?

Coherence is computed from a short stretch of beat-to-beat intervals. The rhythm is broken down into its frequencies, the highest peak in the slow range is found, and the variation packed into a narrow window around that peak is compared with all the rest [S1] [S2]. The more of the rhythm's variation sits in that one peak, the higher the score.

Two things follow from this design. First, the score is a ratio: it describes how concentrated the rhythm is, not how much it varies. Second, the original scoring method comes from researchers at HeartMath [S1] [S2], and other tools use their own formulas. A coherence score from one app or device cannot be compared with a score from another.

Where does it come from?

Coherence is what the body's breathing-linked rhythms look like when breathing is slow and regular. Heart rate rises with each in-breath and falls with each out-breath, the breath-linked heart rhythm. Slowing breathing towards the resonant frequency makes these oscillations much larger [S4], because the breathing cycle starts to work in step with the blood-pressure reflex that steadies the heart. At that pace, breathing, blood pressure and heart rate move in phase [S3]. The mechanism is explained on how breathing changes HRV and is not repeated here.

Coherence is not the same as HRV

Heart rate variability (HRV) describes how much heart rate varies. Coherence describes the shape of that variation: how orderly and single-frequency it is. The two can move separately, and even the authors of the coherence model describe heart-rhythm patterns that change independently of the amount of HRV [S2].

In practice this means:

• High coherence does not mean high HRV. A small, very regular oscillation can score as highly coherent.
• High HRV does not mean high coherence. A large but irregular variation, with several frequencies mixed together, scores low.
• Slow breathing usually raises both during the practice [S3] [S4], which is why the two are often confused. The difference between them, and when each is useful, is covered in HRV vs coherence.

What does the evidence show?

Established. Slow breathing at an individual's resonant frequency produces large oscillations in heart rate and blood pressure [S4], and breathing, blood pressure and heart rate come into step at that pace [S3]. Coherence is a reasonable way to describe that pattern.

Context-dependent. HRV biofeedback, which trains people to breathe at their resonance rate and often uses a coherence-style display, showed a small to moderate effect across randomised studies, and its authors say more research is needed for particular uses [S6]. Details are on the HRV biofeedback page. That evidence is about the training programme as a whole.

Unknown. Whether the coherence score itself, as a separate number, predicts health, emotional state or performance has not been independently established. Much of the research behind the term and its scoring comes from authors affiliated with HeartMath, which sells coherence-feedback devices [S1] [S2]. The claim that specific emotions show up as specific heart-rhythm patterns is the authors' own model [S2].

Is coherence a measure of autonomic balance?

A common claim is that coherence shows the balance between the sympathetic and parasympathetic branches, or that a high score means the two are in perfect balance. Nothing in how the score is built measures either branch: it is a ratio of spectral power [S1]. The peak it relies on sits in the low-frequency range, and a review of the evidence found that this range is mainly determined by the parasympathetic system and that the LF/HF ratio does not work as an index of autonomic balance [S5]. The two branches are covered on the autonomic nervous system page.

Do the heart, brain and lungs lock into the same phase?

During slow breathing, breathing, blood pressure and heart rate do move in step [S3]. The popular extension is that the brain joins in, so that heart, lungs and brain waves synchronise. HeartMath-affiliated authors describe entrainment between the heart and very slow brain rhythms [S2], but this is asserted in a review by authors with a commercial interest, not shown by independent replication. Coherence is measured from the heartbeat alone; it says nothing about brain rhythms.

Does a coherent heart affect other people?

HeartMath research proposes that magnetic fields from the heart may help synchronise heart rhythms between people in a group, and states it as a hypothesis [S7]. Independent evidence that one person's coherence influences another person's body or mind is not established. The wider heart–brain story is on the heart–brain interaction page.

What it does not tell you

• It is a feedback signal during a practice, not a clinical biomarker. It shows how regular your rhythm is right now, while you breathe a certain way.
• It is not your HRV and not your recovery. A high score during slow breathing says little about your resting HRV or how you slept.
• It is not comparable between people or tools. Breathing pace, body size and the scoring formula all shape the number [S4].
• It is not a reading of emotions, stress hormones or brain activity. Vagal tone cannot be measured directly; HRV measures such as RMSSD reflect vagally mediated changes in heart rate.

In ONDA

ONDA shows a coherence score during a practice. It is derived from your heart rhythm and breathing, needs an Apple Watch, and is not available from the iPhone camera [S8]. It is ONDA's own feedback metric, built with its own formula, and is not comparable with HeartMath or other tools; how it is calculated is described on ONDA's method. For practice guidance, see resonance breathing.