A moment to understand · Heart rate variability
The space between beats.
Your heart does not beat like a metronome. The small changes in timing from one beat to the next are called heart rate variability, or HRV. Here is what it is, why your breathing changes it, how it is measured, and what it is not. 75 seconds, narrated, with captions.
Part of A moment to understand, short explainers on the ideas behind my four free brain apps.
What HRV is
Picture the gaps between beats as a row of numbers: 860 milliseconds, then 990, then 930. A healthy heart does not hit the same gap every time. The size of those small, moment-to-moment changes is heart rate variability. It is a different thing from heart rate, which is simply how many beats fit into a minute.
Two branches, one system
The autonomic nervous system adjusts your heart's rhythm automatically. Its sympathetic branch prepares you for action: it mobilizes you for effort, attention and alarm. Its parasympathetic branch supports rest and recovery: digestion, repair and winding down. Both are always working. HRV reflects the ongoing conversation between them.
The vagus connection
The vagus nerve runs from the brainstem down into the chest and belly, and carries parasympathetic signals that slow the heart. Those signals are not constant. As you breathe in, they ease off a little and the heart speeds up; as you breathe out, they return and the heart slows. That breath-linked rise and fall is a large part of what HRV measures, which is why breathing is the practice most closely tied to it.
How it is measured
HRV is calculated from beat-to-beat intervals. An ECG, or a compatible chest strap, records the heart's electrical beats directly and gives the most precise timing. Many watches and rings estimate it instead from light shone into the skin, which works well enough for watching your own trend but is less exact. The number you get also depends on the metric used, how long the recording was, your breathing, and the time of day, so comparing two people's numbers, or two different devices, tells you less than you might hope.
Practice, gently
Research on voluntary slow breathing finds that gentle, slow breathing can increase HRV during the practice itself. My breathing app Allostasis has a few practices that fit:
- Belly BreathingSlow, easy breaths that let the belly rise and fall. A good place to start. Open Allostasis.
- Extended ExhaleAn out-breath a little longer than the in-breath, with no breath holds. Read the guide.
- Humming BreathA long, low hum on the out-breath, which is another way to slow the exhale. Open Allostasis.
Keep breathing comfortable. If a pace feels like effort, slow down or shorten the session. Allostasis is a practice companion; it does not measure HRV itself.
What HRV is not
It is not a diagnosis, and it is not a direct test of how healthy your vagus nerve is. There is no universal good number, and a higher reading on one day does not mean more than a single clue. An increase during a breathing session does not, on its own, show that your resting HRV has changed for good. If you track it, follow your own trend over weeks, in the same conditions, and talk to a clinician about anything that worries you.
Transcript
0:00Your heart doesn't beat like a metronome. The tiny changes in time between beats are called heart rate variability, or HRV.
0:10Your autonomic nervous system adjusts this rhythm automatically. Its sympathetic branch prepares you for action. Its parasympathetic branch supports rest and recovery.
0:21The vagus nerve carries parasympathetic signals that slow the heart. As you breathe, these signals change, helping your heart speed up and slow down.
0:32HRV is calculated from beat-to-beat intervals, measured in milliseconds. An ECG or compatible chest strap records electrical beats. Some watches and rings estimate it using light.
0:46In Allostasis, try Belly Breathing or Extended Exhale. Gentle, slow breathing can increase HRV during practice. Humming Breath offers another way to slow the out-breath.
1:00Keep breathing comfortable. Follow your own trend over time. HRV is one clue, not a diagnosis or a score to chase.
Sources
- Cleveland Clinic: Heart rate variability. Plain-language definition, the two autonomic branches, and electrical measurement.
- Cleveland Clinic: Vagus nerve. Its parasympathetic role, including communication with the heart.
- Shaffer & Ginsberg, 2017. An overview of heart rate variability metrics and norms. Why interpretation depends on the metric, recording conditions and breathing.
- Laborde et al., 2022. Effects of voluntary slow breathing on heart rate and heart rate variability: a systematic review and meta-analysis. Evidence for increased HRV with slow breathing.
- Lehrer et al., 2020. Resonance frequency assessment guidance. Beat-interval measurement and the cardiorespiratory basis of slow breathing.
- 2024 study on longer exhalations. Why this page does not claim a longer exhale beats other slow-breathing ratios.
This page is general information, not medical advice. Written and narrated by Matt Knapp, who runs a brain health program at a medical practice in Ellicott City, Maryland, and builds these apps on the side.
Try it with your breath.
Allostasis is free, runs in your browser, and needs no account.
AllostasisBreathing practice with a visual pacer · allostasis.app
More subjects: A moment to understand · All four apps: knapp.build/brain