What is a good HRV score?

HRV is highly individual. A 'good' score is best defined relative to your own baseline trend. That said, RMSSD above 40 ms is generally associated with good autonomic function for adults under 40. Values decline with age.

Which metric do wearables use for HRV?

Most consumer wearables report RMSSD. Apple Watch reports SDNN. Both are valid metrics but are not directly comparable to each other.

How accurate are wearable HRV measurements?

PPG-based HRV from wrist wearables correlates moderately with ECG (r = 0.60–0.85). Ring devices like Oura generally outperform wrist devices. Sleep measurements are far more reliable than daytime readings.

Heart Rate Variability (HRV) — Definition, Normal Ranges, Device Accuracy

Definition

HRV measures fluctuation in the time intervals between consecutive heartbeats — specifically R-to-R intervals. A heart beating at 60 bpm is not beating with mechanical precision; the intervals vary. Greater variability generally indicates a more adaptive autonomic nervous system.

HRV reflects the balance between sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) nervous system activity. High HRV correlates with strong parasympathetic tone — the body's ability to efficiently regulate stress, recovery, and adaptation.

Why higher isn't always simply "better": HRV is not a universal fitness score. It's a snapshot of autonomic balance shaped by genetics, age, and daily context. A trained athlete and an untrained person can both have healthy, appropriate HRV for their own baseline — what matters most is the trend for your body over weeks, not a single morning's number or how you compare to someone else.

How it is measured

The clinical gold standard is ECG. Consumer wearables use photoplethysmography (PPG) — optical sensors detecting blood volume changes through the skin. The most reported metric is RMSSD (root mean square of successive differences), which emphasizes parasympathetic activity. Apple Watch uses SDNN (standard deviation of all R-R intervals).

Timing matters: HRV readings during sleep are significantly more reliable than daytime readings. Most wearables report overnight averages.

Normal RMSSD values by age and sex

Based on Shaffer & Ginsberg (2017) and Altini & Kinnunen (2021). Values in milliseconds.

Age	Males — Median (ms)	Males — Range	Females — Median (ms)	Females — Range
18–25	55	35–85	60	38–90
26–35	48	28–75	52	30–80
36–45	40	22–65	43	24–68
46–55	33	18–55	36	20–58
56–65	28	15–46	30	16–48
66+	22	10–38	24	12–40

Sources: Shaffer F, Ginsberg JP. Front Public Health. 2017. Altini M, Kinnunen H. JMIR Form Res. 2021.

Which devices measure HRV

Oura Ring

RMSSD — nightly average

Garmin

RMSSD — nightly, used in Body Battery

Whoop

RMSSD — final sleep stage

Apple Watch

SDNN — sleep or on-demand

Polar

RMSSD — highest accuracy PPG

Fitbit

HRV (Premium) — less transparent

What reduces HRV

Alcohol — strong acute suppressor, visible next day
Sleep deprivation and accumulated sleep debt
High training load, overtraining
Chronic psychological stress
Illness and immune activation
Age — HRV declines roughly 2–4 ms per decade
Beta-blockers and anticholinergic medications

What increases HRV

Consistent aerobic fitness training over months
Adequate sleep — quantity and quality
Stress management and recovery practices
Reduced alcohol consumption

Myths vs. reality

Myth	Reality
"A higher HRV number is always healthier."	Only relative to your own baseline. Cross-person comparisons are misleading — age, genetics, and device algorithm all shift the raw number.
"One low reading means something is wrong."	A single night can be skewed by alcohol, poor sleep, illness, or even how a ring or watch fit that night. Trends over 1–2 weeks are far more informative than any single value.
"All devices measure HRV the same way."	Most use RMSSD, but Apple Watch reports SDNN — a different calculation. Scores are not directly comparable across device brands.
"You can meaningfully raise your HRV in a few days."	Short-term changes (better sleep, less alcohol) help, but the larger structural gains come from consistent aerobic training over months.

How each device measures this

Oura Ring

Derives RMSSD from finger-based PPG averaged across the night; ring placement typically yields a cleaner signal than wrist sensors.

Whoop

Captures RMSSD during the final sleep stage before waking; strap must fit snugly for reliable readings.

Garmin

Reports nightly RMSSD and feeds it into Body Battery; daytime stress readings use shorter, noisier windows.

Apple Watch

Uses SDNN rather than RMSSD, so its numbers are not directly comparable to most other brands even for the same person.

Not medical advice: HRV reference data is for educational purposes only. HRV is not a diagnostic marker. Consult a qualified clinician for health concerns.

Heart Rate Variability (HRV)

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What affects this the most?