Heart rate variability (HRV) is the variation in time between one heartbeat and the next, measured in milliseconds. A higher resting HRV usually means the vagus nerve Vagus nerve The main nerve of the parasympathetic nervous system, running from the brainstem to the heart, lungs and gut. Its fast signals to the heart drive most beat-to-beat HRV. More about Vagus nerve is actively slowing and adjusting your heart, which is typical of rest and recovery, but the number only becomes useful when you compare it with your own usual range.
What heart rate variability means
Your heart does not beat like a clock. A heart rate of 60 beats per minute sounds like one beat every second, but the gaps are rarely exactly 1,000 ms. Three beats in a row might be 980, 1,040 and 1,010 ms apart. The time from one beat to the next is called the RR interval RR interval The time from one heartbeat to the next, measured in milliseconds between the R peaks of an ECG. HRV is calculated from a series of these intervals. More about RR interval , after the tall “R” spike on an electrocardiogram ECG A recording of the heart's electrical activity, taken with electrodes on the skin. ECG-based RR intervals are the reference standard for measuring HRV. More about ECG (ECG).
HRV is a summary of how much those intervals change. Two people can both have a heart rate of 60, yet one has gaps that barely move and the other has gaps that drift up and down by 50 ms or more. They share a heart rate but have very different HRV.
An electrocardiogram trace with four tall spikes, the R peaks. The gaps between them are labelled 0.98, 1.06, 0.95 seconds, so no two gaps are the same.
Drag the slider below to see how more or less beat-to-beat change moves two common HRV measures.
- RMSSD
- 30 ms
- SDNN
- 24 ms
- Heart rate
- 60 bpm
An interactive chart of sixteen heartbeats. A slider controls how much the gaps between beats vary. The RMSSD, SDNN and heart rate values update as you move it.
Researchers describe HRV as the change in the time intervals between adjacent heartbeats, produced by several control systems working at once (Shaffer, McCraty and Zerr, 2014). In their words, “a healthy heart is not a metronome”.
Go deeper How the three example beats become an HRV number
Take the gaps from above: 980, 1,040 and 1,010 ms.
RMSSD looks at neighbouring gaps. The differences are +60 ms and -30 ms. Square them (3,600 and 900), average the squares (2,250) and take the square root. RMSSD is about 47 ms.
SDNN looks at the spread of all the gaps around their average. The average is 1,010 ms, so the gaps sit -30, +30 and 0 ms away from it. The standard deviation of those gaps is 30 ms.
Same three beats, two different numbers. A real reading uses hundreds of beats, but the idea is the same. That is why it matters which measure your app shows.
HRV vs heart rate: what’s the difference?
Heart rate counts beats. HRV describes the spacing between them.
| Heart rate | HRV | |
|---|---|---|
| What it measures | How many times your heart beats | How much the time between beats changes |
| Unit | Beats per minute (bpm) | Milliseconds (ms), or a score |
| Usual direction when well rested | Lower | Higher |
| What moves it quickly | Movement, stress, caffeine, fever | Breathing, stress, alcohol, illness, hard training |
The two are linked. When your heart beats more slowly, there is more time between beats and more room for that time to vary. So HRV tends to rise as heart rate falls, partly because of the nervous system and partly because of plain arithmetic (Sacha, 2014). This is one reason a rested morning usually shows both a lower heart rate and a higher HRV. For how the two compare as daily signals, see HRV vs resting heart rate.
What controls HRV: your autonomic nervous system
Your heart has its own pacemaker, a small cluster of cells called the sinoatrial node. Left alone, it would fire at roughly 60 to 100 beats per minute (Shaffer, McCraty and Zerr, 2014). The autonomic nervous system Autonomic nervous system The part of the nervous system that runs the body without conscious control, including heart rate. HRV mostly reflects how it adjusts the heart from moment to moment. More about Autonomic nervous system , the part of your nervous system that runs without conscious effort, constantly adjusts that rate.
The brake and the accelerator
The parasympathetic Parasympathetic nervous system The "rest and digest" branch of the autonomic nervous system. It slows the heart, mainly through the vagus nerve, and drives most of the beat-to-beat variation in HRV. More about Parasympathetic nervous system branch acts like a brake. Its main nerve to the heart is the vagus nerve, which slows the heart. The sympathetic Sympathetic nervous system The "fight or flight" branch of the autonomic nervous system. It speeds up the heart and makes it pump harder, but it acts too slowly to drive beat-to-beat HRV. More about Sympathetic nervous system branch acts like an accelerator. At rest, both are active but the vagal brake dominates (Shaffer, McCraty and Zerr, 2014).
The two branches work at different speeds. Vagal effects arrive in under a second, while sympathetic effects take more than five seconds to build. Because the brake can change the gap between two neighbouring beats, the fast beat-to-beat part of HRV mostly reflects vagal activity Vagal tone The ongoing influence of the vagus nerve on the heart, which slows it down. It is usually estimated from HRV measures such as RMSSD and HF power. More about Vagal tone . This is why RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next. It mainly reflects vagal activity. More about RMSSD , the measure most wearables use, is treated as the main time-based estimate of vagal influence on the heart (Shaffer and Ginsberg, 2017).
Why breathing changes your heartbeat
Breathe in slowly and your heart speeds up a little. Breathe out and it slows down. This rhythm is called respiratory sinus arrhythmia Respiratory sinus arrhythmia The normal rise in heart rate as you breathe in and fall as you breathe out. It is driven by the vagus nerve and makes up much of resting HRV. More about Respiratory sinus arrhythmia , and it is normal (Shaffer, McCraty and Zerr, 2014). It is one of the biggest sources of HRV at rest, which is why breathing pattern matters when you take a reading. To go further, read the nervous system behind HRV.
Why HRV matters
At the population level, lower HRV is linked with poorer health outcomes. A meta-analysis of eight studies with 21,988 people who had no known heart disease found that low HRV was associated with a 32 to 45% higher risk of a first cardiovascular event (Hillebrand and colleagues, 2013). HRV also falls with age and tends to be a little higher in people who are more physically active (Altini and Plews, 2021).
These are group-level associations. They do not mean a low number puts you, personally, at risk. HRV varies widely between healthy people.
What HRV can tell you day to day
HRV is most useful for tracking changes within one person. In a study of 28,175 app users who measured on waking, HRV fell by about 12% after heavy drinking and about 10% during sickness, compared with each person’s own usual values (Altini and Plews, 2021). Hard training, poor sleep and stress can move it too. See what affects HRV for the full list.
Is a higher HRV always better?
Usually, a higher resting HRV than your own normal is a sign of rest and recovery. There are three common exceptions:
- Messy data. Movement or a loose sensor can add false beats or miss real ones. Researchers spend a lot of effort finding and editing these errors because they distort the result (Peltola, 2012). A sudden, unexplained jump is worth checking. See why your HRV is suddenly high.
- Irregular rhythms. Extra heartbeats ( ectopic beats Ectopic beat A heartbeat that starts outside the heart's normal pacemaker and usually comes early. Ectopic beats are common, but they distort HRV unless removed or corrected. More about Ectopic beat ) and atrial fibrillation make the gaps between beats uneven for reasons that have nothing to do with the vagal brake (Peltola, 2012). The number can look high, but that variation is not a sign of good health.
- Other people’s numbers. A higher number than your friend’s says little. Age, genes and the device explain much of the gap.
The lesson on why higher HRV is not always better covers each case, and can HRV detect heart problems explains what HRV can and cannot show about your heart.
How HRV is measured
Every method records the time between beats. The sensor decides how precisely.
| Method | Where it measures | What your app usually shows |
|---|---|---|
| Chest strap (ECG) | Electrical signal from the chest | RMSSD, often in a morning reading |
| Ring, watch or band ( PPG PPG An optical method that shines light into the skin and detects the pulse of blood with each heartbeat. Most watches, rings and phone-camera apps use it to estimate HRV. More about PPG ) | Pulse in the finger or wrist, using light | An overnight RMSSD value, a score or a status label |
| Phone camera (PPG) | Pulse in your fingertip | RMSSD from a short reading |
| Clinical ECG or Holter monitor | Electrical signal, in a clinic or over 24 hours | Several HRV measures, read by a clinician |
Light-based sensors measure the pulse, not the heart’s electrical signal, so strictly they record pulse rate variability Pulse rate variability Variation in the time between pulse beats, measured optically by watches, rings and phone cameras. It closely tracks HRV at rest but drifts from it during movement. More about Pulse rate variability . At rest it agrees well with ECG-based HRV, but movement and some kinds of stress make the two drift apart (Schäfer and Vagedes, 2013).
Devices also report different measures. Most wearables show RMSSD. Apple Watch stores SDNN SDNN The standard deviation of all NN intervals in a recording, in milliseconds. It measures overall variability, and its value depends strongly on how long the recording lasts. More about SDNN , the standard deviation of the intervals between normal beats (Apple Developer Documentation). The two measures give different numbers from the same heartbeats. Read more in how HRV is measured and RMSSD, SDNN and lnRMSSD.
What is a normal HRV?
There is no single normal HRV. It falls with age, and the number depends on the measure, the device and when you measure. Short daytime ECG readings, for example, are not directly comparable with an overnight average from a ring.
Reference values from large studies, split by age and sex and labelled with how they were measured, are on normal HRV by age. For a plain answer to whether your number is fine, read what a good HRV is.
What changes your HRV
Some things you cannot change: age, sex and genes set your usual range. Others move it within that range from day to day:
- Sleep, especially short or broken nights
- Alcohol, which tends to lower HRV the next morning
- Stress, both mental and physical
- Training, especially hard sessions
- Illness, including colds and fever
- Caffeine
- The menstrual cycle, which shifts HRV a little across the month (Altini and Plews, 2021)
If you want to raise your usual level over months, see how to increase your HRV.
What HRV can’t tell you
HRV is not a diagnostic test. A single reading cannot tell you whether your heart is healthy.
- One reading is noisy. Day-to-day swings are normal. Look at the trend over a week or more.
- Devices disagree. Two devices worn on the same night can show different numbers. Compare a device only with itself.
- It does not say why. A drop shows that something is loading your system. It cannot tell a hangover from a cold.
The lesson on building your HRV baseline shows how to read your trend.
Talk to a doctor if you have palpitations, fainting, chest pain or breathlessness, whatever your HRV says.