What Is Heart Rate Variability, and How Should You Track It?

Your watch says your heart rate variability was 42 ms last night and 61 ms the night before. A friend’s says 95. Is yours bad? Probably not. HRV is one of the most personal numbers a wearable gives you, and it only makes sense against your own history, measured the same way every time.

This article is general information, not medical advice. If you’re worried about your heart, talk to a health professional.

What HRV Measures

Your heart doesn’t beat like a metronome. At 60 beats per minute, the gaps between beats aren’t all exactly one second: one might be 0.95 seconds, the next 1.04. Heart rate variability is a measure of how much those intervals change from beat to beat.

Much of that variation comes from your autonomic nervous system. Your heart speeds up slightly as you breathe in and slows as you breathe out, an effect carried mostly by the vagus nerve. When you’re rested, that variation tends to be larger. Stress, illness, short sleep, alcohol, and hard training often make it smaller. That’s why HRV is popular as a sign of recovery.

The standard definitions still in use today come from a 1996 Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology.

SDNN and RMSSD Are Different Numbers

“HRV” isn’t one quantity. There are dozens of ways to summarize the same heartbeats, and wearables use different ones. The two you’ll meet most:

• SDNN is the standard deviation of the intervals between normal beats over a recording. It reflects every source of variation in that period, slow ones included, so it depends strongly on how long the recording is.
• RMSSD is the root mean square of the differences between successive intervals. Because it looks at beat-to-beat changes, it mainly reflects the fast, breathing-linked variation driven by the vagus nerve, and it holds up better over short recordings.

Shaffer and Ginsberg’s review of HRV metrics describes these and many others, and stresses that every value depends on how long and under what conditions it was recorded.

Apple Health stores HRV as SDNN. Oura reports RMSSD, averaged over five-minute samples taken while you sleep. Both are in milliseconds and both are called HRV, but they aren’t the same quantity: an SDNN of 45 ms and an RMSSD of 45 ms say different things. Even two devices that both report RMSSD may measure at different times of night, over different windows, and give different numbers.

Why Comparing With Others Doesn’t Help

HRV varies widely between people. It tends to fall with age, and it differs with sex, fitness, genetics, and breathing. Published norms cover a wide spread, and they depend on the population and the recording method. A low number next to a friend’s tells you little. A drop in your number, compared with your usual, tells you more.

How to Track It Well

1. Measure the same way every time. For most people, overnight readings from the same device are the most consistent, because sleep removes much of the day’s noise: posture, coffee, conversation, exercise. If you take a morning reading instead, take it at the same time, in the same position, before you get up.
2. Stick with one measure. Follow SDNN or RMSSD, usually whichever your device gives. Don’t join two devices’ numbers into one line.
3. Look at averages, not single nights. HRV moves a lot from day to day. Plews and colleagues, who monitor elite endurance athletes, recommend following a rolling weekly average and how much it varies, rather than reacting to one morning.
4. Learn your own baseline. After a few weeks, you’ll see your normal range. A reading well below it, especially several in a row, is the signal worth noticing.
5. Write down what happened. A late dinner, a few drinks, a cold, a long flight, a hard workout. A note turns an unexplained dip into one you understand.

A low HRV isn’t a diagnosis, and a wearable measures it less precisely than an ECG. Use it as a trend to watch, not a verdict.

Tracking HRV in Kaizora

Kaizora keeps the two measures apart on purpose. Heart Rate Variability (SDNN) and Heart Rate Variability (RMSSD) are separate metrics on the Vitals page, so numbers from different methods never end up on one line.

• Import Apple Health. Your Apple Watch’s HRV comes in as Heart Rate Variability (SDNN), along with resting heart rate, oxygen saturation, respiratory rate, sleep, and more. See Importing From Apple Health.
• Log it yourself if your device isn’t imported yet. On Vitals, choose Log and enter SDNN or RMSSD, with the device that measured it. Importing from Oura is coming; until then, you can type in its nightly RMSSD.
• See the trend. On the metric’s page, set the chart to a year and each point becomes a week’s average, which smooths out single nights.
• Keep devices apart. Filter a metric’s page by device to see each one’s numbers on their own. See Add Your Devices.
• Flag dips. Add a reference range around your own baseline, such as a mild band below your usual low, so low readings are flagged on the chart and on Home. See Reference Ranges.
• Compare it. The Correlations page charts HRV beside Time Asleep, Resting Heart Rate, Dietary Alcohol, or your Energy Level. See Comparing Metrics. Lines that move together are a question to look into, not proof of cause.

Resting heart rate often moves the opposite way to HRV, and it’s easier to measure well. See What’s a Normal Resting Heart Rate?

The Short Version

• HRV measures how much the time between heartbeats varies. Higher usually means more rested, but only compared with your own normal.
• SDNN and RMSSD are different quantities. Apple Health uses SDNN; Oura uses RMSSD.
• Measure the same way each time, follow a weekly average, and learn your baseline.
• A single low night means little. Several in a row are worth noticing.

If you want to see your HRV next to your sleep and everything else you track, you can start with Kaizora for free.

Sources

• Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology, “Heart rate variability: standards of measurement, physiological interpretation, and clinical use,” Circulation 93, no. 5 (1996): 1043–1065. doi:10.1161/01.CIR.93.5.1043
• Shaffer F, Ginsberg JP, “An overview of heart rate variability metrics and norms,” Frontiers in Public Health 5 (2017): 258. doi:10.3389/fpubh.2017.00258
• Plews DJ, Laursen PB, Stanley J, Kilding AE, Buchheit M, “Training adaptation and heart rate variability in elite endurance athletes: opening the door to effective monitoring,” Sports Medicine 43, no. 9 (2013): 773–781. doi:10.1007/s40279-013-0071-8
• Apple Developer Documentation, heartRateVariabilitySDNN.
• Oura Help, Heart Rate Variability.