There's no single "good" HRV on Apple Watch. A good reading is one that sits in or above your own usual range, because HRV varies so much from person to person. The number in the Health app is SDNN, measured in milliseconds from short readings your watch takes while you're still. In a large study of healthy adults, average resting SDNN was about 49–50 ms at ages 25–34 and about 28–30 ms at 65–74, with a wide spread around those averages (Voss et al., 2015).
How Apple Watch Measures HRV
Apple Watch records HRV from tachograms: short lists of the time between your heartbeats, captured with the optical sensor while you're sitting or lying still. According to Apple, the watch tries to take one in the background about every four hours and only keeps it if you were still enough for a clean measurement. It samples more often if you've turned on irregular rhythm notifications (every two hours) or AFib History (every 15 minutes). It also records HRV during a Breathe session in the Mindfulness app (Apple, 2024).
From each tachogram, the watch calculates HRV as SDNN, the standard deviation of the intervals between normal heartbeats, and tries to remove irregular (ectopic) beats first. The individual beat-to-beat intervals are saved in Apple Health alongside the SDNN value, so other apps can calculate other HRV measures from them.
Because the watch only keeps readings from still moments, your HRV history in the Health app looks like a scattered handful of readings across the day and night rather than one value per morning, and each one reflects the moment it was taken. If you want a comparable reading each day, do a Breathe session in the Mindfulness app at the same time each morning, since Apple records HRV during those sessions. Wearing your watch snugly, and to bed, gives it more chances at a clean reading.
How accurate is the wrist data? In a study of 20 healthy adults at rest and under mild mental stress, Apple Watch beat-to-beat intervals agreed closely with a Polar chest strap (reliability above 0.9), with a few short gaps per recording (Hernando et al., 2018). The recordings were short and taken in a lab, so the study didn't test overnight use.
SDNN vs RMSSD: Why Apple Watch HRV Doesn't Match WHOOP or Oura
HRV can be calculated several ways, and the two you'll meet most are:
- SDNN (standard deviation of normal-to-normal intervals): how spread out all your beat intervals are over the recording. It reflects your overall variability, and it grows with the length of the recording, so SDNN values from recordings of different lengths shouldn't be compared (Task Force, 1996).
- RMSSD (root mean square of successive differences): how much each interval differs from the one before it. It mainly tracks the fast, parasympathetic ("rest and digest") side of your nervous system, which is why most sports research uses it to monitor recovery (Plews et al., 2013).
WHOOP and Oura both report RMSSD measured during sleep. WHOOP's developer API labels its recovery HRV field hrv_rmssd_milli (WHOOP), and Oura says its nightly HRV comes from rMSSD and is the average of five-minute samples taken while you sleep (Oura).
So an Apple Watch SDNN reading and a WHOOP or Oura number differ in the math, the time of day, and the length of the recording. You can't line them up against each other, and you shouldn't judge an Apple Watch SDNN reading against an age chart built from RMSSD.
What Changed on Apple Watch Series 12 and Ultra 4
With watchOS 27 (September 2026), Apple added more HRV features for Apple Watch Series 12 and Apple Watch Ultra 4. On those models, HRV is measured as often as every five minutes and shown in the Heart Rate app in two forms: Recovery HRV, which Apple says is best for daily signs of stress and recovery, and overall HRV, for broader health. Overnight Vitals now includes Recovery HRV against your personal baseline, and a new Readiness app gives a 0–10 score (Apple Newsroom, 2026).
Apple's announcement doesn't name the formula behind each one. iOS 27 did add an RMSSD data type to Apple Health alongside the SDNN type that has existed since iOS 11 (Apple Developer). If you have one of these watches, check which HRV you're looking at before comparing numbers, because the two variants aren't interchangeable.
Average Apple Watch HRV by Age
One widely cited reference for SDNN comes from a German population study that measured both SDNN and RMSSD in 1,906 healthy adults aged 25 to 74 (Voss et al., 2015). Average values, rounded to the nearest millisecond:
| Age | Women, SDNN | Men, SDNN | Women, RMSSD | Men, RMSSD |
|---|---|---|---|---|
| 25–34 | 49 ms | 50 ms | 43 ms | 40 ms |
| 35–44 | 45 ms | 45 ms | 35 ms | 32 ms |
| 45–54 | 37 ms | 37 ms | 26 ms | 23 ms |
| 55–64 | 31 ms | 33 ms | 21 ms | 20 ms |
| 65–74 | 28 ms | 30 ms | 19 ms | 19 ms |
Compare the number in your Health app with the SDNN columns. The RMSSD columns are there to show how far apart the two measures sit, even in the same people on the same day.
The spread around these averages is wide. In each group the standard deviation of SDNN was roughly 12 to 21 ms, so plenty of healthy people sit well above or below them. Age mattered more than sex: average SDNN fell by about 40% from the youngest group to the oldest, and within each decade the differences between women and men weren't statistically significant.
Keep in mind that these are lab readings. Each value came from a 5-minute ECG taken lying down after 5–10 minutes of rest, while your watch takes its readings with an optical sensor whenever you happen to be still, so your own numbers can run higher or lower. Reviews of HRV norms stress that values from different recording lengths and conditions aren't interchangeable (Shaffer and Ginsberg, 2017).
The study didn't include anyone under 25, so treat these averages as background, not a pass/fail grade. Your HRV is shaped by genetics, fitness, and health as well as age, which is why your own baseline tells you far more than any chart.
Why Is My Apple Watch HRV Low Today?
Start with the reading itself. Apple Watch samples HRV at different moments, so a day whose readings came while you were awake and busy can look lower than one with readings from deep sleep, even if nothing has changed. If your readings usually sit in the high 40s and today's only reading is 31 ms, taken while you were rushing to a meeting, it may say more about the moment than about you. Look at several days before deciding anything.
When the drop is real, these are the usual causes:
- Alcohol. In over 4,000 adults wearing beat-to-beat monitors, alcohol lowered parasympathetic activity during the first hours of sleep in a dose-dependent way, and even low doses had a measurable effect (Pietilä et al., 2018).
- Hard training. After exercise, the parasympathetic system recovers on a timeline that varies a lot between people and can run up to about two days after a session (Stanley et al., 2013).
- Mental stress. In the Apple Watch validation study, RMSSD dropped during a mild mental stress task compared with relaxing (Hernando et al., 2018).
- Short or broken sleep, and getting sick. Both push your nervous system toward "fight or flight", and a dip in HRV alongside a rising resting heart rate is a common early sign that you're coming down with something.
Which Apple Watch HRV Trends Matter
Single readings are noisy, so researchers who use HRV to guide training look at rolling averages over about a week rather than individual days (Plews et al., 2013). Watch for three patterns.
A slow rise over weeks to months, while you train consistently and sleep well, usually means your body is adapting. Several days below your usual range, especially with poor sleep, a higher resting heart rate, or workouts that feel harder than they should, is a reason to ease off for a few days. And a sudden shift right after you change how you measure, such as a new watch, a new HRV variant, or switching between SDNN and RMSSD, comes from the measurement change, so give it a couple of weeks to build a fresh baseline.
HRV isn't a reliable test for overtraining on its own: resting HRV often changes little when athletes overreach (Bellenger et al., 2016). Read it with your sleep, resting heart rate, and how training feels. Our HRV training guide covers how to use those signals to adjust a training week.
When to Talk to a Doctor About HRV
Apple Watch HRV is a wellness measurement, not a diagnosis. Apple describes its Vitals app as "for wellness purposes only and not for medical use" (Apple Newsroom, 2026). Low HRV on its own, without symptoms, is rarely a reason for alarm, but talk to a doctor if:
- You have chest pain, fainting, shortness of breath, or a racing or fluttering heartbeat, whatever your HRV says.
- Your watch sends an irregular rhythm notification.
- Your HRV stays low for weeks along with a resting heart rate that keeps climbing, and rest, sleep, and lighter training don't explain it.
- Your HRV readings jump around erratically from one reading to the next. Apple notes that frequent irregular beats or AFib can affect SDNN when the watch can't filter those beats out, so an unusually high or erratic number isn't always good news (Apple, 2024).
How FITIV Shows Your Apple Watch HRV
FITIV Pulse reads your Apple Watch data from Apple Health and gives you one HRV value per night, measured against your own range rather than a population chart.
The HRV Formula setting (More → Today Tab) decides which HRV that is:
- FITIV (RMSSD), the default: FITIV uses a nightly RMSSD value based on your Apple Watch data. It needs the Beat-to-Beat Measurements permission in Apple Health.
- Apple Health: FITIV uses the HRV samples saved in Apple Health, which for Apple Watch are SDNN.
Because the default is RMSSD from your sleep, FITIV's HRV won't match the number in the Health app, and that's expected. Whichever you choose, the same value feeds your Recovery score, so stick with one formula to keep your baseline consistent.
Where you'll see it:
- Vitals card: on the Today tab, last night's HRV in ms with a status of Typical, Above, Below, Well Above, or Well Below compared with your own typical range from your recent history.
- Vitals Summary: tap the card for your typical range, 7-day and 30-day averages, and a 30-day chart, so you can see trends instead of single nights. More on these screens in Overnight Vitals.
- Recovery: HRV has its own row under Contributors, next to resting heart rate, your Sleep score, and other overnight vitals. See HRV tracking for how it fits together.
Apple Watch, watchOS, and Apple Health are trademarks of Apple Inc. WHOOP is a trademark of WHOOP, Inc. Oura is a trademark of Oura Health Oy. Other product names are trademarks of their respective owners. FITIV is an independent app and has not been authorized, sponsored, or otherwise approved by Apple Inc. FITIV is not affiliated with or endorsed by the other companies mentioned.
Frequently Asked Questions
Is 40 ms a good HRV on Apple Watch?
It depends on your age and your own baseline. In a large study of healthy adults, average resting SDNN was about 45 ms at ages 35–44 and about 37 ms at 45–54, so 40 ms is close to average for many adults. What matters more is whether 40 ms is normal for you: compare it with your usual range over the past few weeks.
Does Apple Watch measure HRV while you sleep?
Yes. Apple Watch tries to take a background HRV reading about every four hours by default, day or night, and keeps it only if you were still enough. It samples more often with irregular rhythm notifications or AFib History turned on. On Apple Watch Series 12 and Ultra 4, Overnight Vitals also shows Recovery HRV against your personal baseline.
Can I convert Apple Watch SDNN to RMSSD?
No reliable conversion exists. SDNN and RMSSD measure different things, and the gap between them changes with age and recording conditions. If you want RMSSD, use a source that calculates it from beat-to-beat data rather than converting Apple's SDNN.
How can I raise my HRV?
Focus on the habits that let your nervous system recover: regular sleep, less alcohol, steady aerobic training with real easy days, and managing stress. Expect changes over weeks to months rather than days, and judge progress by your weekly average, not single readings.
Why did my HRV change when I got a new Apple Watch?
Newer models can measure more often or report a different HRV variant. Apple Watch Series 12 and Ultra 4 show both Recovery HRV and overall HRV, measured as often as every five minutes. Any change in how HRV is measured can shift your numbers, so give yourself a couple of weeks to build a new baseline before comparing.
