Why "What's a Good HRV?" Is the Wrong Question
HRV is one of the most confusing health metrics because the population range is enormous. A healthy 25-year-old elite athlete might have a nightly SDNN of 90 ms. A healthy 60-year-old might have an SDNN of 28 ms. Both numbers can be perfectly normal — for that individual, at that age, with that training history.
This is the core problem with applying population norms to HRV. Unlike blood pressure or cholesterol, where clinical thresholds have clear universal meaning, HRV is best interpreted relative to your own personal baseline. A reading that looks low in isolation might be normal for you. A reading that looks high might be a suppressed value if your baseline is usually higher.
The right question is not "is my HRV good?" — it's "is my HRV above or below my baseline, and has it been trending in a consistent direction?"
What HRV Is Actually Measuring
Heart rate variability measures the variation in time between consecutive heartbeats. If your heart beats at 60 bpm, the average gap between beats is 1,000 milliseconds — but the actual gaps might be 980 ms, 1,020 ms, 990 ms, 1,035 ms. That variation is HRV.
Higher variability generally indicates a healthy, responsive autonomic nervous system where the parasympathetic ("rest and digest") branch is active. Lower variability suggests the sympathetic ("fight or flight") system is dominant — which can indicate stress, insufficient recovery, illness, or physiological aging.
SDNN (standard deviation of normal-to-normal intervals) is the HRV metric Apple Watch uses. It captures overall variability across the entire measurement window. RMSSD (root mean square of successive differences) is more common in research and is more sensitive to parasympathetic activity specifically. Apple Health reports SDNN; many third-party apps convert or also display RMSSD.
How to Establish Your Personal HRV Baseline
Your HRV baseline is a rolling reference point — not a fixed target. Here's how to establish it reliably:
Collect at Least 14–30 Days of Data
A meaningful baseline requires at least two weeks of nightly readings to average out noise from individual bad nights, travel, or acute stress. Thirty days gives you a more stable reference that smooths out weekly rhythms and one-off events.
Measure Consistently — Same Time, Same Conditions
Apple Watch measures HRV automatically during sleep, which is the gold standard for consistency. If you're taking spot readings, morning measurements upon waking (before standing or checking your phone) are the most reproducible.
Note Your Context
Tag known HRV-affecting events: alcohol consumption, intense training sessions, poor sleep, illness, travel across time zones, unusual stress. This context explains deviations from baseline and prevents misinterpretation.
Use a Rolling Average, Not a Fixed Number
Your baseline should update continuously — ideally a 30-day rolling average. As your fitness improves or life circumstances change, your baseline will naturally shift, and your alerts and interpretations should shift with it.
Track Deviation, Not the Number
The most actionable signal is the ratio of your current 7-day average to your 30-day baseline. A ratio above 1.05 (5% above baseline) is a positive signal; below 0.90 (10% below baseline) warrants attention and recovery focus.
The HRV Autonomic Signal computes your 7-day average SDNN, your 30-day baseline, and a baseline ratio. A ratio below 0.90 for more than 3 consecutive days triggers a "declining" status. A ratio above 1.05 signals improvement.
HRV Reference Values by Age and Sex
While personal baseline matters most, population norms provide context — particularly for identifying when a baseline itself is concerning. The table below shows approximate SDNN ranges by age group, cross-referenced against published normative data.
| Age | Males — Avg SDNN | Females — Avg SDNN | Below 20 ms |
|---|---|---|---|
| 20–29 | 55–75 ms | 50–70 ms | Elevated Risk |
| 30–39 | 45–65 ms | 42–60 ms | Elevated Risk |
| 40–49 | 38–55 ms | 35–52 ms | Elevated Risk |
| 50–59 | 32–48 ms | 30–45 ms | Elevated Risk |
| 60–69 | 26–40 ms | 25–38 ms | Watch Closely |
| 70+ | 20–32 ms | 19–30 ms | Watch Closely |
Values are approximate averages from published normative datasets. Individual variation is substantial. SDNN below 19 ms at any age has been consistently associated with elevated all-cause and cardiovascular mortality in longitudinal studies.
Interpreting Deviations from Your Baseline
What Causes HRV to Drop?
Understanding whether a HRV drop is acute (single night) or sustained (multiple days/weeks) changes the interpretation and response completely.
Acute drops (1–3 days)
Alcohol
Even 1–2 drinks measurably suppress HRV the following night, with full recovery taking 24–48 hours. This is one of the most consistent, well-documented HRV suppressors.
High-intensity exercise
A hard training session legitimately drops HRV the following day as the body recovers. This is normal and expected — it should recover within 1–2 days if training load is appropriate.
Poor or short sleep
Insufficient sleep prevents full autonomic recovery, producing a lower HRV the following day. Sleep debt compounds: multiple consecutive poor nights produce a sustained drop.
Illness onset
HRV often drops 12–48 hours before overt illness symptoms appear, as immune activation shifts the autonomic system toward sympathetic dominance.
Chronic drops (weeks or months)
Overtraining syndrome
Sustained high training load without adequate recovery progressively suppresses HRV. Unlike acute training drops that recover within days, overtraining-related HRV suppression can persist for weeks or months.
Chronic psychological stress
Prolonged work, relationship, or financial stress maintains elevated cortisol and sympathetic activity, chronically suppressing HRV. This is one of the hardest HRV drivers to address because it doesn't resolve with a single recovery day.
Cardiovascular aging
Progressive decline in HRV is a feature of cardiovascular aging. A baseline that is consistently declining over months and years — without obvious lifestyle explanation — is a signal worth discussing with a physician.
How to Raise Your HRV Baseline
Consistent aerobic training is the most evidence-backed way to raise HRV baseline over time. Zone 2 cardio (3–4 sessions per week) increases parasympathetic tone and cardiac stroke volume — the two most direct physiological drivers of higher HRV.
Sleep consistency has a significant effect. The circadian rhythm governs autonomic balance; irregular sleep patterns disrupt parasympathetic recovery, keeping the baseline suppressed. A consistent sleep schedule within 30 minutes each night produces measurable HRV improvements within 2–3 weeks.
Stress regulation practices — slow diaphragmatic breathing (4–7 seconds inhale, 6–8 seconds exhale), meditation, and deliberate recovery time — directly activate the vagus nerve and raise parasympathetic tone. Even 10 minutes per day of slow breathing has shown HRV benefits in RCTs.
Reducing alcohol is the highest-return single behavioural change for most people. Even cutting from regular moderate drinking to weekends-only typically produces a measurable HRV baseline increase within 2–4 weeks.
Frequently Asked Questions
What is a good HRV baseline?
There's no universal answer — it depends heavily on age, sex, and fitness level. What matters is your personal 30-day rolling average and whether your 7-day readings are above or below it. For context, SDNN above 50 ms in adults under 50 is generally favorable; below 20 ms at any age is associated with elevated mortality risk.
How does Apple Watch measure HRV?
Apple Watch measures HRV as SDNN during sleep using its optical PPG sensor. It captures readings approximately once per minute during sleep and reports a nightly SDNN value in Apple Health under Heart Rate → Heart Rate Variability.
What causes HRV to drop suddenly?
The most common acute causes are alcohol, a hard training session, poor sleep, and illness onset. A single low reading is less significant than a sustained trend. If your HRV drops for more than 3–4 consecutive days without an obvious explanation, investigate sleep quality, training load, and overall stress levels.
How long does it take to improve HRV baseline?
Most people see meaningful baseline improvement within 4–8 weeks of consistent aerobic training, improved sleep, and reduced alcohol. Elite-level HRV improvement (significant absolute increases in SDNN) typically takes months to years of sustained training.
Is HRV different from heart rate?
Yes. Heart rate measures beats per minute — the average frequency. HRV measures the variation between individual beats. A lower resting heart rate and a higher HRV often go together in fit individuals, but they measure different aspects of cardiovascular and autonomic health and don't always move in the same direction.