Heart rate variability: know your ideal value

Although we talk about a rate of "60 beats per minute", a healthy heart does not beat like a metronome: the time between one beat and the next changes slightly, and that oscillation of a few milliseconds is heart rate variability (HRV). Far from being a flaw, it is one of the subtlest signs that the system regulating the heart is working with flexibility. This article explains what it measures, how it changes with age and sex, where the "reference" figures come from, and why its real value lies not in reaching a perfect number but in knowing your own pattern.

What exactly is heart rate variability?

A healthy heart does not beat identically every second. Between one beat and the next, 920 milliseconds may pass, for example; then 980; then 940. Those small differences are HRV. The time between two beats is called the RR interval, and it is what gets measured and analysed —once any abnormal beats are set aside— to work HRV out.

The sinus node — the heart's natural pacemaker — is acted upon continuously by the two branches of the autonomic nervous system. The parasympathetic branch, chiefly through the vagus nerve, slows the heart within milliseconds and favours rest, digestion and recovery. The sympathetic branch speeds it up more slowly and supports the alert-and-effort response. In practice, HRV is a non-invasive window onto that dialogue: greater variability at rest usually reflects that the resting "brake" (the vagus nerve) is working well, with a good capacity to adapt. It should not be interpreted in isolation or as a diagnosis.

A signal with prognostic value, with caveats

HRV does not measure "how well" the heart beats, but the flexibility of the system that regulates it. Preserved variability reflects that this resting "brake" is working well, with a nimble response to effort, stress or sleep. Persistently low HRV is associated, in many populations, with higher cardiovascular risk and mortality, and it appears early in conditions such as diabetes, heart failure or chronic inflammatory states.

So, is there an ideal value?

The question "what should my HRV be at 40?" has an uncomfortable answer: there is no single correct number, and anyone offering one without conditions is oversimplifying. There are three sound reasons for this.

First: the variation between healthy people is enormous. Two equally healthy people of the same age can differ by more than 100 % in the variability measured. There is no "one size fits all".

Second: the figure depends on how, and how long, you measure. HRV is not a single number: there are several ways to calculate it and they capture slightly different things. And a 24-hour recording, a 5-minute one and a quick reading from a watch are not the same; although they are all called "HRV", they are not interchangeable.

Third: context changes everything. Breathing, posture, time of day, physical fitness, temperature or a recent coffee all alter the figure. There is also a mechanical dependence: the higher the heart rate, the less time between beats and, therefore, the less room to vary. Comparing a number with a table, without fixing all of this, produces false positives and false negatives alike.

What does change with age

The absence of a single value does not mean there are no patterns. What the evidence establishes clearly is that HRV declines with age, and it does so reproducibly. A population study of 1,906 healthy adults, using 5-minute recordings, allows this decade-by-decade fall to be seen in both sexes.

The overall variability (labelled SDNN in the chart) falls from about 50 ms in the thirties to about 29 ms by the seventies. The quick beat-to-beat changes —the ones most tied to the vagus nerve, the "brake" that favours rest (RMSSD in the chart)— fall even more: from about 40 to about 19 ms. The decline is steepest between the ages of 35 and 54 and levels off from 55 onwards. What is more, not everything ages at the same rate: what reflects mainly that resting "brake" is lost earlier and faster than the overall variability.

Women and men do not start from the same point

A meta-analysis of almost 300,000 healthy people described a consistent pattern: women have, on average, a slightly faster heart and slightly lower overall variability, but a greater weight of the "resting" system (the vagal brake). Men show a slightly more "alert" profile despite their lower rate. That contrast is summed up by an indicator of the balance between the two systems —labelled the LF/HF ratio in the chart—: that difference between men and women tends to shrink after the age of 55.

Why your watch may show different figures

HRV is not a single piece of data. Each app may show you a different figure and call it "HRV", and they do not mean the same thing. It is worth knowing which one your watch is showing you:

Hence a simple rule: do not compare one device's figure with another's. Although both are called "HRV", they are not measuring the same thing in the same way. What helps is to follow your own figure, on the same device, over time.

Where the "reference" cut-off points come from

The best-established clinical use of HRV comes from assessing risk after a myocardial infarction, using 24-hour Holter recordings. That is where the thresholds come from —based on the overall variability (SDNN)— that are sometimes quoted as if they applied to everyone.

For short recordings in healthy people — the most common scenario — aggregated reference values do exist. A systematic review of 44 studies (21,438 adults) provides these orders of magnitude for 5 minutes.

The width of the ranges across studies illustrates, better than any argument, why it is unwise to fix a rigid "target": even among healthy adults the normal figures overlap enormously.

How to measure it so the data are comparable

1. Always use the same device and the same figure. Do not compare one ring's reading with another watch's score: they are not equivalent.
2. Measure under repeatable conditions. On waking, before coffee, exercise or stressful news, and in the same posture. Another valid option is to use the same device's overnight average.
3. Avoid interpreting recordings taken while moving. HRV needs to detect each interval between beats precisely.
4. Record the context. Sleep, alcohol, training, infection, stress, medication and the menstrual cycle help to explain the changes.
5. Watch over weeks, not minutes. A single measurement carries a great deal of biological and technical noise; it is best to build the baseline over two to four weeks of consistent measurements.

What can make your HRV fall?

HRV changes naturally, and a fall does not automatically mean illness. Among the factors that can reduce it temporarily are insufficient sleep, a training load greater than can be recovered from, alcohol consumption, fever or an infection, sustained stress, pain, dehydration and some medications. It also declines with age, though with wide variation between people.

How to interpret your trend without alarm

From the clinic to the wrist: what smartwatches measure

For decades, measuring HRV required a Holter monitor and a laboratory. Today, smartwatches and rings estimate it daily, which puts the measurement within everyone's reach and calls for sensible interpretation.

Most read the pulse with a light on the skin (a technique called photoplethysmography, or PPG), not the electrical trace of the electrocardiogram. At rest and with a regular rhythm it agrees well with the electrocardiogram. These devices focus on the quick beat-to-beat changes because they are the most reliable measure when the recording lasts only a few minutes. They tend to measure while you sleep: the resting "brake" is stronger at night, so a night-time reading in calm conditions gives a cleaner baseline. Even so, the light on the skin is sensitive to movement, to how the device sits on the wrist and to each brand's calculations.

Can HRV be improved?

There is no quick trick or supplement that guarantees a high HRV. The most sensible strategies are the same ones that support cardiovascular health and recovery: tailored aerobic and strength exercise, sufficient sleep, less alcohol, stress management, adequate hydration and treatment of underlying medical problems. Slow breathing can raise HRV during the session itself because it alters the interaction between breathing, blood pressure and heart rate, but it is worth distinguishing between temporarily changing the measurement and improving long-term recovery capacity.

Conclusion: your "ideal value" is a pattern, not a figure

HRV has no magic number by age. It has a terrain — one that descends predictably over the years, with a slightly different profile in women and men — and a personal history that, for the first time, can now be followed day by day. Its greatest usefulness is not in saying whether today is a pass or a fail, but in showing how the body responds over time, when it is measured consistently and combined with information on sleep, training, stress and symptoms.

HRV works like a compass, not a diagnosis. And that heart which refuses to be a metronome is, precisely, the good news.

Sources and further reading

• Task Force ESC/NASPE: heart rate variability — standards of measurement and interpretation (Circulation, 1996)
• Shaffer & Ginsberg: an overview of heart rate variability metrics and norms (Frontiers in Public Health, 2017)
• Nunan et al.: normal values for short-term HRV in healthy adults (Pacing Clin Electrophysiol, 2010)
• Voss et al.: short-term HRV — influence of gender and age, KORA cohort (PLoS ONE, 2015)
• Umetani et al.: 24-hour HRV over nine decades of life (J Am Coll Cardiol, 1998)
• Koenig & Thayer: sex differences in healthy human HRV — a meta-analysis (Neurosci Biobehav Rev, 2016)
• Kleiger et al.: decreased HRV and mortality after myocardial infarction (Am J Cardiol, 1987)
• Rodrigues et al.: HRV monitoring using commercial wearable devices (Sensors, 2022)

This article is for information only and does not replace individual assessment or advice from a healthcare professional. For acute symptoms such as severe chest pain, call 112.