What is HRV, and what is a good number?

Heart rate variability (HRV) is the small change in time from one heartbeat to the next, measured in milliseconds.^[1] There is no single good number: what is normal for one person may not be for another, and HRV tends to fall with age.^{[2, 1]} Because readings swing from day to day, an average of several mornings is more useful than any one number.^[5]

What HRV measures

Your heart does not tick like a clock. Even at rest, the gap between beats changes by a fraction of a second.^[2] A healthy heart shows this kind of change all the time.^[1]

Heart rate variability (HRV)

The variation in the time between heartbeats, in milliseconds (ms). Heart rate counts beats per minute (what a good resting heart rate is). HRV looks at how uneven the spacing between those beats is.

HRV reflects the nervous system that runs your body in the background. The most common app measure, RMSSD, mostly tracks the "rest and digest" side, carried by the vagus nerve.^[1] It is not a pure readout of that system, though. Breathing faster or slower can change RMSSD on its own.^[1]

The numbers your app shows

You may see RMSSD or SDNN, and they measure different things.^[1] The length of the recording matters too: numbers from recordings of different lengths are not comparable.^[1]

How RMSSD is calculated

RMSSD comes from the gaps between beats. You take each gap minus the one before it, square those differences, average them and take the square root.^[1] The worked example uses five made-up gaps so you can follow the math.

What is a normal HRV?

There is no single healthy number. In a review of 44 studies with 21,438 people, some studies found very large person-to-person differences, mostly in frequency-based measures.^[3] As summarized in a later review, its average short-term RMSSD was 42 ms, with a range of 19 to 75 ms across the studies.^[1]

Age matters. HRV measures fall as people get older.^[1] In data from 8,203,261 Fitbit users, HRV measures dropped with age and changed strongly with the time of day.^[4] Fitness matters too: HRV tends to rise with aerobic fitness.^[1]

What moves your HRV

• Alcohol. In 4,098 Finnish workers, RMSSD in the first 3 hours of sleep fell by 2.0 ms after low intake, 5.7 ms after moderate intake and 12.9 ms after high intake.^[8]
• Short sleep. A review of 11 trials found RMSSD fell after sleep loss, though the authors call the effect relatively modest.^[9]
• Stress. In most studies, HRV changed under stress, but the changes were not consistent from study to study.^[18]
• Hard training. Intense exercise generally lowers vagal HRV for 24 to 48 hours.^[5]
• Illness. In 297 health care workers wearing a watch, the daily HRV pattern differed in the week around a COVID-19 diagnosis.^[13] Only 13 were diagnosed, and the authors say this limits how well HRV can predict infection.^[13]

Can you raise your HRV?

Regular exercise training tends to raise resting HRV. A review of 13 studies found a moderate rise in a vagal HRV measure (effect size d=0.48).^[10] The same review found an even larger drop in resting heart rate.^[10]

Slow breathing raises HRV while you do it. Most studies find the biggest effect near six breaths per minute.^[11] A review of 223 studies found higher HRV during slow breathing, right after it, and after programs of several sessions.^[12]

Lower HRV has been linked to a 32% to 45% higher risk of a first cardiovascular event in people without known heart disease.^[14] That is a link seen in groups, not proof. Whether raising your HRV lowers your risk is still an open question.^[1]

How to measure it well

Sports scientists use this routine with athletes:^[5]

1. 1Measure soon after you wake up, lying down or seated, in the same spot most days.^[5]
2. 2Stay still and breathe normally. Short-term numbers assume normal breathing, about 11 to 20 breaths per minute.^[1]
3. 3Endurance athletes are advised to take readings at least 3 to 4 mornings a week and average them.^[5]
4. 4Expect large day-to-day swings, so judge the average, not one morning.^[5]

Phone cameras and chest straps both agreed with an ECG for RMSSD in a study of 29 healthy people at rest.^[6] Sensors that read your pulse through the skin are less reliable when you move or are under some kinds of stress.^[7]

When to talk to a clinician