What Is HRV? Heart Rate Variability Explained Simply
Short answer: HRV (heart rate variability) is the variation in time between your heartbeats, measured in milliseconds. Your heart does not tick like a metronome: at 60 bpm one gap might be 950 ms and the next 1,050 ms. At rest, more variation usually means your body is relaxed and recovering, and less variation means it is under strain from stress, poor sleep, illness or hard training.
This guide explains what HRV measures, the two numbers you will see (RMSSD and SDNN), what is normal for your age, and how to measure it.
HRV is a wellness signal, not a diagnosis. If you have symptoms such as chest pain, fainting or palpitations, see a doctor.
What HRV actually measures
Your heart rate is set by two branches of the autonomic nervous system pulling in opposite directions:
- The sympathetic side ("fight or flight") speeds the heart up and makes the beats more regular.
- The parasympathetic side ("rest and digest"), mainly through the vagus nerve, slows it down and lets each beat shift slightly, for example speeding up a little as you breathe in and slowing as you breathe out.
HRV is a window into that balance. When you are rested and calm, the parasympathetic side has room to work and HRV is high. When you are stressed, ill, short on sleep or still recovering from a hard workout, the sympathetic side dominates and HRV falls.
RMSSD vs SDNN: the two numbers you will see
| RMSSD | SDNN | |
|---|---|---|
| What it measures | How much each beat-to-beat interval differs from the last | The overall spread of all intervals |
| What it mostly reflects | The parasympathetic ("recovery") branch | Total variability, both branches |
| Who reports it | WHOOP, Oura, Garmin, most HRV apps | Apple Watch (Apple Health "Heart Rate Variability") |
| Typical resting adult value | Tens of ms, falls with age | Tens of ms, usually a bit higher than RMSSD |
The two are related but not interchangeable: an Apple Watch HRV of 40 ms and a WHOOP HRV of 40 ms do not mean the same thing. Compare a device only with its own history. (Since iOS 27, Apple Health can also store RMSSD from the newest watches; see Apple Watch HRV.)
What is a normal HRV?
Age matters more than anything else. Typical resting RMSSD:
| Age | Typical RMSSD | Healthy range |
|---|---|---|
| 18-25 | ~68 ms | 55-105 ms |
| 26-35 | ~58 ms | 40-90 ms |
| 36-45 | ~48 ms | 30-75 ms |
| 46-55 | ~38 ms | 25-60 ms |
| 56-65 | ~30 ms | 20-50 ms |
| 65+ | ~25 ms | 18-45 ms |
More detail, by sex and percentile, in Good HRV by Age.
Even within an age group, healthy people differ a lot. A systematic review of 44 studies and over 21,000 healthy adults found very large differences between individuals, which is why your own baseline matters more than any chart. A value that is normal for you and steady over weeks is a good sign; a sustained drop below your usual range is the signal to pay attention to.
Why HRV matters
- Recovery and readiness. HRV is lower the morning after hard training, alcohol or a bad night, and recovers as you do. Many athletes adjust training on low-HRV days.
- Early warning. A drop of several ms below your baseline, often with a higher resting heart rate, can show up a day or two before a cold.
- Long-term heart health. In a meta-analysis of 8 studies and nearly 22,000 people without heart disease, those with the lowest HRV (SDNN) had about a 35% higher risk of a first cardiovascular event than those with the highest.
What lowers and raises HRV
| Lowers HRV | Raises HRV over time |
|---|---|
| Short or poor sleep | 7-9 hours on a regular schedule |
| Alcohol, especially in the evening | Regular aerobic exercise |
| Illness and inflammation | Recovery days after hard training |
| Hard training without recovery | Slow breathing (around 6 breaths a minute) |
| Stress and anxiety | Losing excess weight |
| Dehydration, late heavy meals | Less alcohol |
| Age | Managing stress |
See How to Increase HRV for what moves it most.
How to measure HRV
- Chest strap + app: the most accurate, because it records every heartbeat electrically.
- Smartwatch or ring: automatic, mostly overnight or during rest, good for trends.
- Camera spot check: your phone or laptop camera can pick up your pulse from your face or fingertip. Try our free online heart rate monitor for heart rate, HRV and a stress estimate in 45 seconds.
Whatever you use, measure the same way at the same time, ideally right after waking, and judge the 7-day average rather than one reading.
Low HRV: when to take it seriously
A single low reading is normal. Take notice if your HRV stays well below your usual range for more than a week without an obvious cause, especially with a rising resting heart rate, unusual tiredness or poor sleep. Very low HRV together with symptoms such as palpitations, dizziness or chest pain needs a doctor, not an app.
Track HRV the useful way
HRV on its own is a noisy number. Next to your resting heart rate, sleep and training it becomes useful: it tells you when to push and when to rest.
Bodly reads your HRV, resting heart rate and sleep from your Apple Watch every day and turns them into one recovery score, alongside your weight, meals and progress photos. It is free to use, no trial needed.
Keep reading
- Good HRV by Age: What's Normal?
- How to Increase HRV
- Apple Watch HRV: What's Normal and How Accurate?
- Resting Heart Rate by Age
Sources
- Task Force of the ESC and NASPE. Heart rate variability: standards of measurement, physiological interpretation and clinical use. Circulation (1996)
- Shaffer F, Ginsberg JP. An overview of heart rate variability metrics and norms. Front Public Health (2017)
- Nunan D, et al. A quantitative systematic review of normal values for short-term heart rate variability in healthy adults. Pacing Clin Electrophysiol (2010)
- Hillebrand S, et al. Heart rate variability and first cardiovascular event in populations without known cardiovascular disease: meta-analysis and dose-response meta-regression. Europace (2013)
Frequently asked questions
Heart rate variability (HRV) is how much the time between your heartbeats changes from one beat to the next, measured in milliseconds. A heart beating at 60 bpm does not beat exactly once a second: one gap may be 0.95 s, the next 1.05 s. More variation at rest usually means your body is relaxed and recovering well.
At rest, generally yes: a higher HRV than your usual baseline points to good recovery and low stress, and across large studies people with low HRV have a higher risk of heart disease. But HRV differs a lot between people, so compare it with your own baseline and your age group rather than with someone else.
It depends mostly on age. Using RMSSD, a typical resting value is around 58 ms at 26-35, 48 ms at 36-45, 38 ms at 46-55 and 30 ms at 56-65, with wide healthy ranges around each.
Both measure variation between beats. RMSSD looks at changes from one beat to the next and mainly reflects the relaxing (parasympathetic) side of the nervous system; most recovery wearables report it. SDNN is the overall spread of beat intervals; Apple Watch has reported SDNN. The numbers are not interchangeable.
Short or poor sleep, alcohol, illness, hard training without recovery, stress, dehydration and ageing. A sudden drop from your normal is often the first sign of an oncoming cold or overtraining.
A chest strap with an app is the most accurate. Smartwatches and rings measure it automatically, mostly overnight. You can also get a spot reading from your phone or laptop camera, which works best when you sit still in good light.
Related reading
Apple Watch Sleep Score: How It Works and What's a Good Score
How Apple Watch calculates your Sleep Score (duration 50, bedtime 30, interruptions 20), what each range means, why it can feel too generous, and how to raise each part.
Resting Heart Rate by Age: Chart for Men & Women
What is a normal resting heart rate for your age and sex? Percentile charts from 14,000 healthy adults, what counts as good, when to see a doctor, and how to lower yours.
Apple Watch HRV: What's Normal and How Accurate?
What a normal Apple Watch HRV looks like (with data from 61 users), how the watch measures it, why it reads lower than WHOOP or Oura, how accurate it is and how to use your trend.