Your heart doesn’t beat like a metronome. The interval between each beat varies slightly — by tens of milliseconds, beat to beat. The interval shortens slightly when you inhale, lengthens slightly when you exhale, fluctuates with emotion, attention, posture, and effort.
That variation is heart rate variability. HRV. The number most wearables now surface as a daily score. HRV Breathe (by SUUR) is built around it — we read it from Apple Health before and after every session so you can see whether the practice is actually working.
Why a varying heartbeat is a good thing
Counterintuitively, a steady heart rate is a sick heart rate. A completely regular pulse — every beat the same distance from the last — is what you see in patients with severe heart failure, advanced diabetes, or extreme stress. A healthy heart adapts continuously to what the body needs. Bigger swings between beats reflect a more responsive autonomic nervous system.
The autonomic nervous system has two opposing branches. Sympathetic is the gas pedal — fight, flight, focus, urgency. Parasympathetic is the brake — rest, digest, recover, restore. The balance between them shifts constantly. HRV reflects how quickly that balance can shift.
SDNN and RMSSD — which one is on your screen?
There are several ways to compute HRV from a stream of heartbeats. Two are common in consumer devices:
- SDNN — Standard Deviation of Normal-to-Normal intervals. The basic statistical spread of the beat intervals. Apple Watch reports SDNN, and HRV Breathe reads it from Apple Health.
- RMSSD — Root Mean Square of Successive Differences. Better captures the short-term variation driven by the vagus nerve. Oura, Whoop, and most overnight HRV measurements use RMSSD.
The two metrics correlate, but their absolute values differ — which is why your Apple Watch HRV number is different from your Oura ring number. Trends matter more than absolute values. Compare yourself to yourself.
What lowers your HRV
- Acute and chronic stress
- Poor sleep, particularly broken sleep
- Alcohol — even one drink lowers HRV measurably for 24+ hours
- Illness, especially viral infection (often the first signal)
- Overtraining or insufficient recovery
- Dehydration
- Anxiety, particularly anticipatory anxiety
What raises it
- Slow breathing at the body’s resonance frequency (~6 breaths per minute) — the most reliable intervention, and what HRV Breathe is built around
- Aerobic fitness (long-term)
- Consistent, adequate sleep
- Cold exposure (brief)
- Vagus nerve activation broadly — humming, gargling, slow exhalation
- Time outdoors
- Active emotional regulation
How wearables actually measure it
Different wearables sample your heart at different points and compute HRV differently. Knowing how yours works helps you read the number honestly.
- Apple Watch.Optical PPG sensor at the wrist, sampled in 30–60 second windows triggered by the Breathe app or background sampling. Reports SDNN. Quality is high when you’re still, lower during movement. Watch must be snug.
- Oura ring, Whoop, Garmin overnight. Continuous PPG during sleep, integrated across the whole night. Reports RMSSD. The most stable HRV measurement available to consumers — overnight removes most of the daytime noise (stress, posture, caffeine).
- Chest straps (Polar H10, etc.). Electrical ECG, the same modality as a clinical Holter monitor. Gold standard for accuracy but inconvenient. Best for one-off measurements (HRV biofeedback sessions, baseline calibration).
- Phone camera apps (Welltory, HRV4Training). PPG via the rear camera, 60-second hold. Lower noise floor than wrist PPG; meaningfully accurate if you do it the same way every morning at the same time.
None of these are wrong, but their numbers don’t directly compare. Pick one, stick with it, and compare your seven-day trend to your seven-day trend a month ago. That’s the only comparison the underlying physiology supports.
Why a single day’s HRV is mostly noise
HRV varies dramatically within a person from day to day. A 15–25 percent swing between consecutive overnight readings is normal, even when nothing material has changed in your life. Factors that move a single day:
- Yesterday’s training load (even moderate cardio drops it)
- How much you ate, and when (late dinners suppress overnight HRV)
- Room temperature (cooler rooms = higher HRV)
- Hydration status
- Whether you woke once or twice in the night
- Caffeine timing — coffee after 2pm shows up in your data
- Any acute stress event in the prior 12 hours
This is why a seven-day rolling average is the only readout worth checking. Daily numbers are signal-plus-noise; weekly averages strip out most of the noise and reveal the underlying trend. If you check your HRV daily and try to interpret single-day movements, the most likely outcome is you’ll convince yourself an intervention worked when it didn’t, or didn’t work when it did.
When a sudden HRV drop matters
Most single-day drops mean nothing. But a sustained drop — say, 15–20 percent below your seven-day baseline for two to three days running — is one of the earliest physiological signals you get of a developing illness. Apple, Whoop, and Oura have all published validation studies showing HRV declines 24–48 hours before symptomatic onset of viral infection in a meaningful fraction of users.
It’s not a diagnostic. You can have a low HRV week and be fine; you can be developing flu and have a normal HRV. But if the trend is meaningfully down for several days and you can’t explain it through obvious behavior (training, late nights, alcohol), it’s a soft signal to take it easy for 48 hours. Most people regret pushing through that signal.
What HRV is not
It’s not a measure of cardiovascular fitness (use VO₂ max or resting heart rate). It’s not a measure of stress directly (it’s a measure of autonomic flexibility, which correlates with stress recovery). It’s not a diagnostic tool for medical conditions. And it’s not a number you should chase compulsively — the act of obsessing over your daily HRV will lower it.
Use it as a slow signal. Check the trend weekly, not hourly. Pair it with one intervention you can actually do — slow breathing, sleep, less alcohol — and watch the trend move.