Breathing ThatKnows If It's Working

Vayu reads heart rate variability from your Apple Watch or Wear OS smartwatch after each session, guides you into coherent breathing with haptics, and shows you whether you actually got there. No guesswork. No screen. Just your wrist.

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SFU Metacreation Lab

Research partner

199 participants

Pilot study cohort

What Is HRV Coherence?

HRV coherence is the state in which heart rate rises and falls in a smooth, regular wave in step with the breath, usually at five to six breaths per minute. It reflects strong parasympathetic engagement and baroreflex activity, and it is the pattern resonance-frequency breathing is designed to produce.

Your heart doesn't beat like a metronome. The tiny variations between beats, called heart rate variability (HRV), are a direct window into your autonomic nervous system. High HRV means your parasympathetic (rest-and-digest) system is active. Low HRV means you're in a stress state.

Coherence is a specific HRV pattern that occurs when your heart, lungs, and nervous system synchronise. It typically happens when you breathe at your resonance frequency, roughly 5–6 breaths per minute for most adults. In this state, your HRV waveform becomes smooth and rhythmic, your blood pressure regulation improves, and your stress hormones drop measurably.

Traditional breathing apps just set a timer. Vayu reads the HRV your watch records after each session to see whether your nervous system is actually responding, and uses that to shape the haptic guidance in your next session, so you can find and hold your coherence zone over time. To make sense of the number it reports, see what counts as a good HRV by age. For running this at organisation scale, see HRV wellness programs for teams.

How Vayu Uses HRV on Your Wrist

Reads your watch sensor

Reads the HRV your Apple Watch (Series 6+) or Wear OS watch records from its optical heart rate sensor after each session. No extra hardware needed.

Tracks your HRV response

Compares each session's HRV with your baseline to see whether your practice is moving you toward a coherent state, the goal of breathwork protocols like box breathing and coherent breathing.

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Guides with haptics

Taps your wrist on each inhale and exhale at a coherent-breathing pace, shaped session by session by your HRV. No screen, no audio. Just your watch and your breath.

Is HRV biofeedback evidence-based?

Yes. Meta-analyses of controlled trials found that heart rate variability biofeedback training reduces self-reported stress and anxiety (Goessl, Curtiss and Hofmann 2017, 24 studies) and improves emotional and physical outcomes more broadly (Lehrer et al. 2020, 58 studies), with the largest effects when resonance-frequency breathing is practised regularly over weeks.

Most of those trials used chest-strap or finger sensors with a live display. Vayu differs: it reads HRV from the wrist optical sensor after each session and shapes the next one, and its own evidence is the uncontrolled pilot below. Sources are listed at the end of this page.

The Evidence

What Vayu's own pilot found, with the caveats that make the numbers honest.

+28.6%

median HRV improvement

In Vayu's 199-participant pilot, median HRV rose +28.6% over 4 to 6 weeks among the 29% who kept up three or more sessions a week. Uncontrolled, under peer review.

64%

30-day retention

Day-30 retention reached 64% in our strongest 2025 cohort. It is the number we watch, because HRV only moves for people who keep practising.

−7.4 bpm

resting heart rate

Median resting heart rate change in the same pilot cohort, alongside a −16% drop in perceived stress (PSS-10, 2.5 points).

199

pilot participants

Built with the SFU Metacreation Lab and supported by NRC IRAP. Vayu itself has passed 70,000+ downloads in 15+ countries.

See full evidence page →

Compatible Devices

Apple Watch

Series 6, 7, 8, 9, 10, Ultra, Ultra 2
SE (2nd gen)
watchOS 10.6 or later

Wear OS

Google Pixel Watch 1/2/3
Samsung Galaxy Watch 4/5/6/7
Fossil Gen 6+
Wear OS 3.0 or later

FAQ: HRV Biofeedback Breathing

What is HRV biofeedback breathing?

HRV biofeedback breathing uses your own heart rate variability data to guide your breathing practice. By breathing at the right pace (usually 5-6 breaths/minute), you can synchronise your heart rhythm and nervous system into a state called "coherence." The feedback shows you whether your breathing is actually achieving this effect, not just timing it.

How does Vayu use HRV biofeedback?

Vayu reads your heart rate variability from your Apple Watch or Wear OS smartwatch after each session and uses it to shape the next one. During a session, haptic pulses on your wrist pace your inhale and exhale; afterwards, you see how your HRV responded and the next session adapts to it. Over weeks, that record confirms whether your breathing is actually shifting your nervous system, not just a timer.

What is coherent breathing?

Coherent breathing (also called resonance frequency breathing) means breathing at the pace that maximises your heart rate variability, typically around 5 breaths per minute (5-5 or 4-6 inhale/exhale ratio). At this pace, your cardiovascular, respiratory, and nervous systems synchronise, producing a measurable reduction in stress and anxiety.

Is HRV breathing evidence-based?

Yes. HRV biofeedback breathing is supported by peer-reviewed research. Studies published in Applied Psychophysiology and Biofeedback show significant reductions in anxiety, blood pressure, and cortisol with regular practice. The HeartMath Institute has over 30 years of research on HRV coherence techniques.

Do I need special equipment for HRV breathing?

With Vayu, you only need an Apple Watch Series 6+ or a Wear OS 3.0+ smartwatch (Pixel Watch, Galaxy Watch 4/5/6/7, Fossil Gen 6+). Vayu uses the optical heart sensor you already have on your wrist. No chest strap or separate device needed.