The vagus nerve, HRV and slow breathing

Almost everything on this site comes with a caveat about contested evidence. This article is the exception, and it is worth knowing why.

Respiratory sinus arrhythmia

Your heart rate is not constant. It rises slightly as you inhale and falls as you exhale — a normal, healthy pattern called respiratory sinus arrhythmia, mediated largely by the vagus nerve acting as a brake on the heart's pacemaker.

The size of that beat-to-beat variation is heart-rate variability, and it is commonly summarised as RMSSD — the root mean square of successive differences between heartbeats. Higher generally indicates a more responsive autonomic system.

None of this is fringe. It is standard physiology, measurable on consumer wearables.

The resonance effect

Here is the useful part. Breathe at roughly six breaths a minute — about a five-second inhale and a five-second exhale — and the oscillation in your heart rate becomes unusually large.

This happens because at around that rate the natural rhythm of breathing lines up with the slower rhythm of blood-pressure regulation. The two reinforce rather than interfere, and the resulting swing in heart rate is substantially bigger than at any other rate. It is called the resonance frequency, it varies a little between people, and it sits near six breaths a minute for most adults.

That is a mechanical, reproducible effect. You can watch it happen on an HRV app in about two minutes.

Why this matters for a sound company

Because the infra-low sessions in this catalog — 0.1 Hz and below — are not really entrainment sessions at all. They are breath pacers.

A 0.1 Hz rhythm is one cycle every ten seconds: six breaths a minute. Inhale as the field rises, exhale as it falls.

• 0.1Hz Resonance Pace (6 BPM) — 25 minutes at the classic rate.
• 4-7-8 Parasympathetic Exhale — 0.08 Hz, weighted to a long exhale.
• Box Breathing 4x4 Pacer — even phases, better for alertness than for winding down.

You are not waiting for audio to do something to you. You are using it as a metronome for something you are doing. That is why it works regardless of what you believe about entrainment — and why it is the first thing we would suggest to a sceptic.

Lengthen the exhale. Vagal activity increases during exhalation, so an exhale longer than the inhale amplifies the effect. A 4-second inhale and a 6-second exhale is a good starting ratio.

About polyvagal theory

You will encounter Stephen Porges's polyvagal theory attached to almost every discussion of the vagus nerve — ventral and dorsal vagal states, neuroception, the social engagement system. It has been enormously influential in therapy.

It is also genuinely contested in the scientific literature, and you should know that before adopting its vocabulary. Grossman and Taylor have argued in detail that respiratory sinus arrhythmia is too confounded by breathing parameters to serve as a clean index of vagal tone, and that the theory's evolutionary account of vagal control is not accurate. Later critiques challenge its core premises directly; defenders have responded; the debate is unresolved.

Our position: the underlying physiology — vagal control of heart rate, RSA, the resonance effect — is solid and is what these sessions rely on. The larger theoretical superstructure is interesting and disputed, and we are not going to present it as settled to make a sound file seem more profound.

What we are not claiming

That audio stimulates your vagus nerve directly. You will see this asserted for low-frequency sound, on the basis that a branch of the vagus innervates part of the ear canal. That anatomical fact is real. The leap from it to "playing 60 Hz through headphones is vagus nerve stimulation" is not established, and transcutaneous VNS research uses electrical current, not sound.

What the breath sessions do is pace your breathing. The breathing does the rest. That is a smaller claim and a true one.