HRV GUIDE

What is HRV, and what does it tell you?

Heart rate variability, or HRV, is how much the time between your heartbeats changes from one beat to the next. It’s very personal: healthy people can have very different numbers, and HRV tends to fall with age. The useful question isn’t whether your number is “good,” but how it compares with your own usual.

What is heart rate variability?

Your heart doesn’t tick like a metronome. Even at a steady 60 beats per minute, one gap between beats might be a little longer than a second and the next a little shorter. HRV measures that variation, in milliseconds.

It reflects how your nervous system is balancing “go” and “rest” signals. Harvard Health explains that when your body is in more of a fight-or-flight state, the variation tends to be lower, and when you’re more relaxed, it may be higher.

Why does HRV vary so much between people?

There’s no single normal number. A 2025 systematic review of HRV reference values concluded that there are no generally accepted normal values yet. HRV depends on age, sex, fitness, time of day, and the way it’s measured.

In a study of more than 8 million Fitbit users, HRV declined with age and shifted strongly across the day, and more active people tended to have higher HRV. So a number that’s typical at 25 can be well above what’s typical at 60, and both people can be perfectly healthy.

What are SDNN and RMSSD?

They’re two ways of doing the math on the same heartbeats.

SDNN looks at how spread out all the gaps between beats are over a stretch of time. Apple Watch records HRV in Apple Health as SDNN.
RMSSD looks at how much each gap differs from the one just before it. It mainly reflects the calming, rest-and-digest side of your nervous system. Some other wearables report RMSSD, and Apple Health on iOS 27 has a separate place for it.

The two aren’t interchangeable, and recording length matters too: longer recordings tend to show higher HRV. Only compare a number with others measured the same way, ideally on the same device.

Why compare with your own baseline?

Because the spread between people is so wide, comparing your HRV with a friend’s, or with a chart online, says very little. Your own history is a far better yardstick. Researchers who track HRV in elite athletes rely on averages over several days and on each athlete’s individual pattern, rather than on single readings.

A change that holds for several days, compared with your usual range, is more meaningful than any one number.

What can lower HRV?

Alcohol. In a study of more than 4,000 people, HRV in the first hours of sleep was lower after drinking, and more alcohol meant a bigger drop.
Stress and poor sleep. Harvard Health lists both among the things that can keep your body in fight-or-flight mode.
Illness. HRV changes have been linked with infection. In a study of health care workers wearing Apple Watch, HRV patterns shifted around a COVID-19 diagnosis.
Hard training. A tough stretch of exercise can lower HRV for a while, though in well-trained athletes the pattern isn’t always simple.
Age. HRV gradually declines over the years.

Is a higher HRV always better?

Not automatically. Across large groups, higher HRV tends to go with fitness and good health. But for one person on one night, a big jump isn’t necessarily good news. A widely cited review of HRV measures notes that some heart conditions, such as atrial fibrillation, can make HRV look high. In elite athletes, both rises and drops in HRV have been seen when training wasn’t going well.

A reading far outside your usual range, in either direction, is worth a second look rather than a celebration.

Does one night’s HRV mean much?

Not much on its own. Overnight HRV moves with breathing, sleep stages, what you did that day, and how many readings your watch caught. Apple says Apple Watch samples HRV while you’re still, so the number of readings can vary. Look at the pattern across a week or more before drawing conclusions.

When should you talk to a doctor?

HRV from a watch isn’t a diagnostic test. Whatever it says, talk to a doctor if your pulse feels irregular or seems to skip beats, or if you have palpitations, dizziness, fainting, chest discomfort, or shortness of breath. If you’ve noticed a lasting change in your readings and you don’t feel like yourself, that’s worth mentioning too.

How Wellplease helps

Wellplease reads overnight HRV from Apple Health and compares it with your own usual range, using up to 60 earlier days once it has at least 7 prior nights. HRV is one part of your Recovery score, alongside sleeping heart rate, last night’s sleep, and the past week’s sleep. SDNN and RMSSD are never mixed. If your watch doesn’t share HRV with Apple Health, as with Garmin, Recovery uses sleeping heart rate and sleep instead. The scores are wellness estimates, not medical measurements, and your health history stays on your iPhone.

Sources

This guide is general information, not medical advice. Last reviewed October 10, 2026.

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