Heart rate variability (HRV): what it is, values by age and why it changes so much

Heart rate variability, HRV for short (VFC in Spanish), is the number on your watch that confuses people most. Resting heart rate explains itself: 58 is 58. HRV shows up as 25 ms one night and 48 the next, painted red or green, without anyone explaining why. This text covers what it measures, what RMSSD and SDNN mean, which values are usual by age and sex according to the largest cohorts, why two watches give such different numbers, what lowers it and how many days it takes to come back, and how to read yours without comparing it with anyone else.

One thing before we start. HRV does not detect arrhythmias and does not replace any heart test. If you have palpitations with fainting or near-fainting, chest pain or pressure, shortness of breath, or your watch repeatedly warns you of an irregular rhythm, the curve will not answer you: chest pain or pressure is a reason to call emergency services, and the rest, a reason to consult without waiting.^[10]

What HRV is and what its numbers mean

Your heart does not beat like a metronome. If your pulse is 60 per minute, the average interval between beats is one second, but one beat arrives at 0.92 seconds and the next at 1.08. HRV is exactly that: the fluctuation in the time intervals between consecutive heartbeats.^[1] It is measured in milliseconds (ms): the higher the number, the more the rhythm varies from one beat to the next.

That variation is produced by the autonomic nervous system, the one that regulates what you do not control. Its parasympathetic branch, the vagus nerve, slows the heart and dominates when you rest, digest and sleep; the sympathetic branch speeds it up when action is needed. The most used metric, RMSSD, mainly reflects vagal activity, and the high-frequency band (HF) reflects parasympathetic activity and the rhythm of breathing.^[1] In plain terms: a higher HRV usually indicates a body in recovery mode, and a lower one, a body spending on something: effort, alcohol, fever or worry.

That is why “higher is always better” is a simplification. Today’s HRV depends on how you breathe (slower, deeper breaths raise it), on posture, on how much you moved before measuring it, on your age and on your sex.^[1] And the index many apps present as the “balance” between the two branches, the LF/HF ratio, has an interpretation that specialists themselves describe as controversial.^[1] HRV is a window into the state of your nervous system at that moment, not a health grade.

RMSSD, SDNN and the frequency metrics

In your watch’s app you may run into these abbreviations. The first two are the ones that matter at home:

• RMSSD: the root mean square of the differences between successive intervals. It is the main measure of vagally mediated HRV,^[1] and the one many watches, rings and apps use.^[3]
• SDNN: the standard deviation of all the intervals between normal beats in the period measured.^[1] It is what Apple Watch reports.^[7] Over 24 hours it captures everything that happens in the day; over five minutes, much less, which is why it is not correct to compare a five-minute SDNN with a 24-hour one.^[1]
• LF and HF: the low-frequency (0.04 to 0.15 Hz) and high-frequency (0.15 to 0.40 Hz) bands. HF reflects parasympathetic activity and breathing; LF is harder to interpret.^[1]
• Recording length: five minutes, a whole night or 24 hours are not interchangeable; they use the same formulas, but their physiological meaning can differ profoundly.^[1]

HRV values by age: the table

Honesty first: no cardiology society has published an official table of “normal HRV by age”. What exists are large cohorts of healthy people with medians and percentiles by age and sex. The largest is the Dutch Lifelines cohort: 84,772 adults aged 13 to 91, without cardiovascular disease, hypertension, type 2 diabetes or obesity, and without certain medicines that alter HRV, measured with a ten-second resting electrocardiogram.^[2]

Two warnings. These are ten seconds at rest, in the daytime, with electrodes, not your whole night with an optical sensor on your wrist; your watch’s numbers will be different, often higher, because HRV rises at night.^[1] And what the table teaches is not your numbers but the shape of the curve and the width of the range. The values are the median RMSSD in milliseconds and, in parentheses, the 2nd and 98th percentiles: the interval that holds 96 % of healthy people of that age.^[2]

Three things stand out. The slope: the median falls steadily from age 20 to 60 and then levels off.^[2] The width: at 40 to 44, 96 % of healthy women fall between 9.7 and 123.7 ms.^[2] Two equally healthy people of the same age can have 12 and 110, which is why your coworker’s number says nothing about yours. And the lower limit stays remarkably stable across the lifespan:^[2] what changes with age is mostly the headroom above. Sex weighs little: between 20 and 45 women have a median about 5 ms higher, and the difference disappears after 60.^[2]

Other reference points you will see quoted: in 24-hour Holter recordings, an SDNN below 50 ms has been classified as “unhealthy” and above 100 ms as “healthy”, cut-offs born in cardiac patients that do not carry over to a watch’s nightly reading.^[1] In recordings of about five minutes, a review of 44 studies with 21,438 healthy adults found a mean RMSSD of 42 ms, with a range of 19 to 75.^[1]

Why your watch and someone else’s give such different numbers

It is the most frequent question and it has three answers that add up: the device, the measurement window and the person.

The device and the formula

An electrocardiogram records the electrical signal of the heart; most watches and rings use an optical sensor that estimates the beat from changes in light reflected by the skin. When seven consumer devices and apps were compared against a resting electrocardiogram, the error in RMSSD was 4 % in the best case, 7 to 11 % in the others and over 100 % in an app that used the phone camera.^[3] Chest straps with electrodes were more accurate than optical sensors, and the length of the recording weighed more than the type of technology.^[3]

Add the formula: some devices report RMSSD, others SDNN, and they do not give the same number in the same person.^[1][7] And each manufacturer chooses the window: the whole night, deep sleep, the minutes before waking or five minutes after getting up. Night recordings are in theory the most standardized condition, but they are noisy and hard to implement daily, which is why sport has preferred a 5 to 10 minute measurement on waking.^[6] Two watches of different brands on the same wrist can show 35 and 55 on the same night, and both be working fine.

Night versus day

HRV has its own 24-hour rhythm: the vagal band (HF) tends to rise at night and fall during the day.^[1] That is why your nighttime reading is higher than a midday one, and a table built from ten seconds of daytime rest will never match your watch at 7 in the morning. Always compare the same window: night with night, morning with morning.

Why it is not compared with someone else’s

Go back to the table: among healthy people of the same age and sex the range runs from under 10 to over 100 ms.^[2] Add that age explains a good part of the difference,^[1][2] that breathing changes the result^[1] and that the device, the formula and the window are not the same.^[1][3] Your partner having 70 and you 28 says nothing about who is healthier. The only thing that makes sense is to compare your HRV with your HRV: same watch, same wrist, same window, over weeks. It is the idea behind why the reference range is not your personal norm, applied to the watch.

What lowers HRV and how many days it takes to come back

This is the useful part of the metric. HRV responds to what you do and what happens to you with fairly predictable logic. The table summarizes what studies measured for each situation and how long it took to return; where there is no number, it is because the literature does not give one.

Alcohol is the most frequent cause of a “bad night” on the curve and the least suspected. The low dose in the study went up to 0.25 grams of alcohol per kilogram of body weight, little more than one glass of wine for a 70-kilogram person, and heart rate during sleep rose even with that amount.^[4] If you add a late, heavy dinner to the drink, nighttime digestion piles on; in sleepiness after eating we explain what happens to glucose in those hours.

Infection is where HRV has the most practical value and where it is easiest to get scared. In health care workers wearing Apple Watch, the SDNN pattern changed before diagnosis and before symptoms, and between 7 and 14 days later it was no longer distinguishable from an uninfected person.^[7] That is the scale of a mild infection: one or two weeks. For a strong one, the scale is different: in 234 people with confirmed COVID-19, resting heart rate took 79 days on average to return to its level, sleep 24 and steps 32.^[8] If your resting heart rate is still five beats above your own a month after the flu, tell your doctor; the table is in normal resting heart rate by age.

Training is the easiest cause to recognize and the one most often read backwards. A hard session lowers HRV for one or two days:^[6] it is the expected response, not a warning. What deserves attention is that it does not return when load is reduced,^[6] or that it drops together with a higher resting heart rate, worse sleep and tiredness that does not go away. If that is your case, read tired all the time: which tests to ask for before the appointment.

Stress is the hardest to measure and the thing people look for most in HRV. The review of 37 studies concludes that HRV is affected by stress and supports its use as an objective measure, but warns that the changes were not uniform and that many physical conditions and habits influence the result.^[9] Translated: a sustained low HRV during a hard stretch is coherent, but proves nothing on its own, and an app that tells you “your stress is 72” is interpreting more than the science allows. If the stretch comes with brain fog or higher blood pressure than usual, measure it at home with the protocol in normal blood pressure and bring both together.

How to read it: your seven-day average and your range, not one night

HRV is noisy by nature. In athletes monitored daily, the day-to-day coefficient of variation of RMSSD sits between 10 and 20 %, and in short resting measurements around 12 %, against 82 % for the LF/HF ratio.^[6] In other words, even if nothing in your life changes, tomorrow’s RMSSD can be 15 % below today’s. Here is how to read it well:

1. Build your baseline: two to four weeks with the same watch, on the same wrist, with the same window (night or morning). In sport, months or years of follow-up are needed to know a person’s profile.^[6]
2. Look at the seven-day average, not the night, and compare it with the previous seven nights and with last month. A weekly average that drops 10 % or more and stays there is a signal; a night that drops 25 % and comes back is not.
3. Know your range: the lowest and the highest night of a normal month. A reading within that range is your normal, even if the app paints it red.
4. Note the events: alcohol, late dinner, hard training, bad night, travel, onset of symptoms. Without events the curve cannot be interpreted; with them, it almost always explains itself.
5. Measure how many days you take to come back. It is the most personal number in HRV: if after a drink you return in one night and after the flu in ten days, you already know how you recover.
6. Read HRV together with resting heart rate and sleep. A low HRV with a high heart rate and short sleep tells a coherent story; a low HRV alone is usually noise or the sensor.

And when to stop observing and consult: if the weekly average has been below your range for two or three weeks with no event to explain it, and there is also tiredness that does not improve with rest, a resting heart rate higher than yours or palpitations, take the log to your doctor. Not because HRV diagnoses anything, but because it is organized context that helps decide which tests to order.

Warning signs

HRV does not detect arrhythmias and cannot rule them out. The symptoms that demand attention are the usual ones, with or without a watch. According to the American Heart Association, an isolated premature beat is felt as a palpitation or a skipped beat, a “fluttering” in the chest is associated with atrial fibrillation, and when an arrhythmia lasts long enough to affect how well the heart works, more serious symptoms appear.^[10] Consult without waiting if you have:

• Palpitations with fainting or near-fainting, dizziness or lightheadedness.^[10]
• Chest pain or pressure: it may be a heart attack, call emergency services immediately.^[10]
• Shortness of breath together with a fast or irregular heartbeat.^[10]
• Rapid heartbeat or pounding in the chest, or a pulse that alternates fast and slow.^[10]
• A very high or very low resting heart rate that is sustained, several days outside your range, especially with tiredness or weakness.^[10]
• Repeated irregular rhythm notifications on the watch: they are not a diagnosis, but they are a reason for your doctor to check with an electrocardiogram.^[10]

What to bring to the doctor

A screenshot with a number in red tells your doctor nothing. This does:

1. Weekly HRV averages for the last one or two months, with the name of the metric (RMSSD or SDNN) and the window (night or morning); without that the numbers cannot be interpreted.^[1]
2. Your resting heart rate for the same period and your hours of sleep: these are the three curves read together.
3. The list of events with dates: alcohol, intense training, trips, start and end of symptoms, and how many days you took to come back each time.
4. Medicines and supplements you take, because some modulate HRV and heart rate.
5. Specific symptoms: palpitations and when, dizziness, tiredness, changes in sleep, mood.
6. Your latest results, if you have them: a complete blood count, thyroid panel and glucose are usually ordered when tiredness and heart rate do not add up; in complete blood count and thyroid panel we explain what they include.