300+ answered questions, ESC & AHA sourced

Honest answers about
heart-rate tracking on your iPhone.

Is iPhone HRV measurement accurate enough to trust? Can a phone really replace a Whoop strap? What does Apple Health hand off, and what's yours alone? The medically-sourced answers — without the marketing.

HRV accuracy Blood pressure Apple Health Privacy Subscription
Most-asked question

"Is HRV measurement from an iPhone camera actually accurate?"

For RMSSD and resting heart rate, iPhone photoplethysmography (rear camera + flash on a fingertip) tracks within roughly ±3 ms of a chest-strap ECG when you sit still for 60 seconds. That's clinically acceptable for trend monitoring — which is what HRV is for. It is not a diagnostic device, and we never claim it is.

Sourced — Plews et al., 2017 · ESC 2021 Prevention Guidelines
Median error vs ECG
±3 ms
Clinically acceptable
Measurement time
60 sec
Same as a Polar H10
Extra hardware
None
Your iPhone is the sensor
Studied since
2013
PPG-on-smartphone literature
Browse by topic

Pick a topic. Get a straight answer.

Measurement

How the camera reads your pulse, how long, what stops a reading.

8 answers →

HRV & recovery

RMSSD vs SDNN, what your trend means, when to back off training.

6 answers →

Blood pressure

Cuff readings, AHA stages, why iPhone can't read BP from a finger.

5 answers →

Apple Health

What syncs in, what syncs out, and how to fix duplicate readings.

4 answers →

Privacy

Where your readings live, what we can and can't see, full export.

4 answers →

Subscription

What's free, what's Plus, family sharing, cancel anytime.

3 answers →

Measurement

8 questions
Do I need to keep the screen on during a reading?
Yes — Pulse needs the flash on, which means the screen stays on. We dim it to minimum brightness during a reading to save battery (a full 60-second measurement uses about 0.4% of an iPhone 15 battery).
My reading failed — why?
Three usual suspects: your finger is too cold (capillary circulation drops), pressure is too firm (squeezing the capillaries closed), or you moved. Warm hands, light contact, and a still arm fix 95% of failed readings.
Can I take a reading after coffee?
You can — but it won't be your "true resting" HRV. Caffeine suppresses parasympathetic tone for 60–90 minutes. For a comparable trend, measure at the same time of day, ideally before coffee. Pulse will tag a post-caffeine reading so you can spot it in your trend later.
Does Pulse work on older iPhones?
iPhone 8 (2017) and newer. Anything with a True Tone flash and a rear camera that supports 30fps video capture. SE models work but are slightly noisier — we recommend 75 seconds instead of 60 on those devices.

HRV & recovery

6 questions
My HRV dropped 20% overnight — should I be worried?
One reading is noise. A 20% one-day dip is well within normal biological variation — alcohol the night before, poor sleep, fighting off a virus, or just measurement variability all cause it. Worry only if the dip persists for 5+ consecutive days below your 30-day baseline.
Is a high HRV always better?
No — and this is the most-misunderstood HRV fact. HRV is relative to you. A 30-year-old endurance athlete's "normal" might be 80 ms; a healthy 60-year-old's might be 35 ms. Both are fine. What matters is your personal trend, not the absolute number.
RMSSD vs SDNN — which should I watch?
For day-to-day recovery: RMSSD. It reflects parasympathetic ("rest & digest") tone and responds within hours to training, sleep and stress. SDNN reflects overall autonomic balance and moves on a weekly timescale. Pulse plots both; your Wellness Index blends them.
Why is my HRV higher on rest days?
Because your nervous system has had time to recover. That's the whole point of the metric. A rising HRV on Tuesday after a hard Monday session means you're adapting; a falling HRV on Tuesday means the load was too much for your current fitness.

Blood pressure

5 questions
Can Pulse measure my BP from my fingertip?
No — and any app that claims it can is misleading you. Blood pressure requires a cuff: it's a measurement of the force against your artery walls and the only way to capture it is to compress them. Pulse logs readings from your cuff (or Apple Health), classifies them by AHA 2017 stages, and shows the trend. The measuring stays with the cuff.
Which BP cuffs sync into Pulse?
Anything that writes to Apple Health does. Omron, Withings, QardioArm and iHealth all sync automatically. For non-smart cuffs (most pharmacy models), the in-app keypad takes about 4 seconds to log a reading.
What stage am I in if my readings are 132 / 84?
Stage 1 hypertension, per AHA 2017. Pulse uses the higher of the two numbers to classify, so 132 puts you in Stage 1 (130–139 systolic) regardless of the diastolic. That's not a crisis — but it's worth discussing with your doctor and measuring more consistently.

Apple Health

4 questions
What does Pulse read from Apple Health?
Workouts (for TRIMP scoring), resting heart rate (for cross-validation), blood pressure (for stage classification), and sleep duration (for HRV context). Nothing else — and we never write back to Apple Health unless you explicitly toggle "Save reading to Health" on a measurement.
Will my Apple Watch HRV show up?
Yes, but it's labelled separately on the chart. Apple Watch measures HRV opportunistically during sleep, which is a different physiological state from a morning resting reading — they don't replace each other. Many users find the combination more informative than either alone.

Privacy

4 questions
Where do my readings live?
Locally on your iPhone, in the encrypted app sandbox. If you turn on iCloud sync, they sync end-to-end encrypted across your own Apple devices. Auxility servers never see your raw readings — we receive only aggregated, anonymous usage telemetry, which you can disable in Settings → Privacy.
Can I export everything?
Yes. Settings → Export gives you a single PDF report (great for sharing with your doctor) and a CSV of every reading you've ever taken. We don't gate exports behind Plus — your data is yours.

Subscription

3 questions
What's the difference between free and Plus?
Free gives you unlimited HRV, RHR and BP logging, the Wellness Index, and Apple Health sync. Plus ($3.99/mo or $24.99/yr) adds the 30/90-day Progress charts, TRIMP training-load scoring, PDF reports, and Family Sharing for up to 5 people.
Can I cancel anytime?
Yes — through the App Store like any iOS subscription. Your data stays; only the Plus-only views are hidden. Re-subscribe and it all comes back.

Best answered with the app open.

Most of these questions evaporate after your first reading. Pulse is free to download.

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