Do I need a chest strap or an extra device to measure HRV?

Not for LeibFit. The app measures nothing itself; it reads the HRV your watch already writes to Apple Health. A chest strap does deliver cleaner raw data by design, but it is not necessary for comparing against your own norm — as long as you always use the same device.

Why does LeibFit show different HRV values than other apps?

Because "HRV" can mean different metrics. The Apple Watch writes SDNN to Apple Health; other systems work with metrics such as RMSSD and sometimes measure over a different time window. The numbers cannot be converted into one another. Only the trend within one system is meaningful.

What is a good HRV value?

That question cannot be answered in general. HRV depends on age, genetics, time of day and measurement method, and is not comparable between two people. This is why LeibFit works exclusively with the distance to your own 30-day norm instead of reference tables.

Can I measure HRV with the iPhone alone, without a watch?

No. Overnight HRV happens at your wrist, not in your phone. Without a wearable that measures at night and writes to Apple Health, there is no basis for recovery, sleep or load — which leaves only the training side of LeibFit, and that alone does not justify the app.

Does my health data leave the iPhone?

No. LeibFit reads from Apple Health and calculates on the device. There is no account, no server and no analytics SDK inside the app. Your values go nowhere, and you can export your data at any time.

HRV app for iPhone — without extra hardware

Most people looking for an HRV app have been wearing the measurement hardware for a while already. What is missing is not another device, but something that puts the number in context. That is where LeibFit starts: the app measures nothing itself. It reads what your watch writes to Apple Health and works out an interpretation on your iPhone.

Where HRV on the iPhone comes from

The chain is short and verifiable at every link: your watch measures the intervals between heartbeats overnight and stores an HRV value in Apple Health. LeibFit asks Apple Health for that value — after you explicitly grant access — and compares it with your own values from the past 30 days. There is no intermediate step through a server, because there is no server.

SDNN, not RMSSD — the most important sentence on this page

"HRV" is not a single number but a family of metrics. The Apple Watch writes SDNN to Apple Health: the standard deviation of beat-to-beat intervals across a measurement window. Another widespread metric is RMSSD, which puts more weight on the differences between consecutive beats.

Both describe the same underlying phenomenon, but with a different calculation and on a different order of magnitude. They cannot be converted into one another. So if someone tells you their system shows an HRV of 65 while your app shows 38, that is not a statement about the two of you — it is a statement about two different metrics.

We spell this out because it is the honest way to describe an independent measurement basis: different, not worse or better.

Why you should not compare HRV numbers across apps

Different metric. SDNN and RMSSD produce different numbers for the same night.
Different time window. Averaging over a few minutes of deep sleep or across the whole night changes the result considerably.
Different measurement site and optics. Wrist, finger and chest strap do not measure equally cleanly.

The practical consequence: stick with one system and watch the trend. Switching devices effectively resets your basis for comparison.

What to look at instead

Not the absolute value, but the distance from yourself. LeibFit builds a personal norm from your past 30 days and uses it to answer the only question a watch can answer seriously: is today's value above or below what is normal for you? HRV, resting heart rate and sleep together produce a recovery score from 0 to 100 — guidance for your training, not a diagnosis.

Which watches work

All of those that write their values to Apple Health. The Apple Watch does so directly. Other manufacturers usually offer a sync option inside their own app, where the transfer to Apple Health has to be switched on. LeibFit itself talks only to Apple Health and to no manufacturer account.

Only one thing really matters: the watch has to be worn at night. HRV, resting heart rate and sleep all arise exactly then.

When LeibFit is the wrong app for you

You never wear a watch at night — then the basis for any recovery analysis is missing, not just for this one.
You use Android — LeibFit is iPhone only.
You are looking for medical readouts such as ECG interpretation or blood pressure analysis — LeibFit explicitly does not do that.

What happens beyond the HRV number

A pure HRV readout would be a number without consequence. In LeibFit the training planning hangs off it: a daily load score is calculated on a scale from 0 to 100 out of heart rate zones and training volume, today's target follows your recovery, and a due weight increase is postponed when recovery is poor — with a reason given. On top of that, a morning journal shows you after a few weeks which factors correlate with your recovery and how strongly. Correlations, not causes.

Guidance for your training, not a medical diagnosis.