Everything you need to know about HRV

Heart rate variability (HRV) is how much the time between your heartbeats changes from one beat to the next, in milliseconds. It runs highest when you’re rested. Judge yours against your own recent nights on the same device.

That last part is where most people go wrong. An Apple Watch and an Oura ring give different numbers for the same night, and two healthy people can sit far apart. Your own last few weeks are the only fair comparison.

The short version

• What it is. At 60 beats a minute your heartbeats aren’t one second apart. Some gaps are a little shorter and some a little longer, and HRV measures how much they change. What HRV is.
• Why the gaps change. The vagus nerve works like a brake on your heart that can press and let go within one beat. Each breath in eases it and each breath out presses it, so a rested heart speeds up and slows down with every breath. What makes it vary.
• Why it matters. In 808 people who had survived a heart attack, those with the lowest HRV died at 5.3 times the rate of those with the highest. In healthy people it’s a modest sign of future risk, and an early sign that something changed last night. Why it matters.
• How it’s measured. Your device times each pulse, then summarizes the gaps. Apple Watch reports SDNN, Oura, WHOOP and Garmin report RMSSD, and each measures over a different stretch of time. How to measure it.
• How to know if yours is low. Build your own normal from two to three weeks of nights, then look at the 7-day average. A week below your usual band matters. One low night usually has a cause you can name. Your own normal.
• How to raise it. Finish hard workouts and dinner a few hours before bed, drink less and earlier, and train steadily for months. How to raise it.

Isn’t a steady heartbeat the healthy one?

It seems like it should be. A heart that beats like a clock sounds reliable, and the word variability sounds like a fault.

At rest it’s the other way round. A heart that ticks evenly at rest is usually under some kind of load, such as drinks, a hard workout close to bed or an illness. Age evens it out too. A rested heart is uneven in a regular way, faster on each breath in and slower on each breath out. As one review of the field puts it, “A healthy heart is not a metronome.”

This isn’t the same as an irregular rhythm such as atrial fibrillation, where the beats come chaotically. That can make HRV look very high, and an ECG is what tells the two apart.

A simple way to picture it

Walk beside a friend while you talk. You speed up a little, slow down a little, and keep adjusting to stay in step. Now run for a train. You go flat out, and all the small adjustments disappear.

Your heart at rest is the walk. The small adjustments are HRV. Anything that pushes the body toward the train, from a few drinks to a cold coming on, shrinks them.

How did we start measuring it?

The first person to describe it was Stephen Hales, the English clergyman who measured a horse’s blood pressure with a tube in its artery. In 1733 he reported that the gap between beats, and the pressure itself, changed as an animal breathed. In 1847 the German physiologist Carl Ludwig recorded it on a smoked drum in dogs: the pulse quickened on each breath in and slowed on each breath out.

For more than a century that was a curiosity. Then in 1965 the researchers Hon and Lee noticed that the variation between an unborn baby’s heartbeats shrank before signs of distress, even before the average heart rate changed. Fetal heart monitoring is now standard care.

In 1987 a team led by Kleiger looked at 24-hour recordings from 808 people who had survived a heart attack, and followed them for about two and a half years. Those whose SDNN was under 50 ms had a death rate 5.3 times that of those over 100 ms. In 1996 a European and North American task force set the standard definitions of SDNN and RMSSD.

The newest chapter is the wrist and the finger. Oura alone now measures HRV every night for 6 million members, and that’s where most of today’s confusion comes from.

What changed since our old guide

Gyroscope’s first HRV guide said some things we no longer stand behind.

• It isn’t a balance meter. The old guide described HRV as the balance between the body’s gas pedal and brake. In 2013 the physiologist Billman showed that the ratio used to measure that balance doesn’t do so, and a 2026 guideline paper from a group of heart researchers says HRV shouldn’t be used as a marker of it. Beat-to-beat HRV mostly tracks the brake.
• It doesn’t measure your brain or your immune system. HRV can change before and during an illness, but that’s a change in how the heart is being driven, not a measure of immune strength. A normal number rules nothing out.
• REM isn’t where HRV runs highest. The old guide had it the wrong way round for the measures most devices use. In sleep lab studies, the brake side of HRV was strongest in deep sleep, and REM looked more like being awake.
• One morning doesn’t decide your day. In a study of 10 runners over nine weeks, the change in their weekly average HRV tracked how much their 10 km time improved, and single days barely did.

The sections

What does your device’s number mean?

Each device has its own number and its own words for it.

What good looks like

There’s no single good number. Across 6 million Oura members in 2026 the average was 41 ms, and the median was 36 ms for women and 35 ms for men. Garmin’s own guide says a normal range for one person isn’t the same as for someone else. HRV falls with age and rises with fitness, and every device has its own scale.

So the useful questions are about you. Is this week lower than your usual? Is your resting heart rate up too? Did something happen last night? Your own normal has the method.

What the evidence doesn’t show

• That raising your number lowers your risk. Low HRV goes with more heart disease, but no trial has shown that pushing it up prevents any.
• That HRV can rule out a heart problem. If you have chest pain, faint, or feel your heart racing or skipping, see a doctor, whatever your device says.
• That any readiness score has a tested cut-off. The training trials we cite set their own rules, such as a drop below a 10-day average, and didn’t test a wearable’s readiness score.
• That two devices can be merged. SDNN and RMSSD summarize the beats in different ways, and the same SDNN can come with very different RMSSD values, so there’s no formula between them.

Track it in Gyroscope

Connect your ring or watch, and Gyroscope shows every night’s HRV against your own normal, with your sleep and drinks right beside it. Apple Watch’s SDNN and the RMSSD from Oura, WHOOP and Garmin are kept apart and never averaged. See tracking it in Gyroscope.