Why SDNN instead of population standards?

SDNN is the primary time-domain measure of heart rate variability that research links to stress load and recovery capacity. But a “normal” range differs enormously between a 25-year-old athlete and a 60-year-old office worker. Tuning anchors every reading to your own 14-day baseline, so a value that looks “average” in a population table can still signal a meaningful shift for you.

How does the 14-day rolling baseline work?

Tuning only ever stores your own SDNN samples. Day 3 gives an initial assessment, Day 7 establishes a formal baseline, and from Day 14 onward a sliding 14-day window continuously re-baselines. Your readiness is the distance between today’s samples and your personal window — never a fixed population norm.

Does Tuning simulate workout recovery?

No — this is our core principle. We never fabricate or extrapolate recovery curves. If Apple Health records a new SDNN sample, the chart moves; if it doesn’t, nothing is drawn. What you see is always the real signal from your watch.

Is my health data secure and private?

Yes. 100% of HealthKit data processing happens on-device. Tuning never uploads, stores, or shares your health data — there is no account database of your readings, and nothing ever leaves your iPhone.

How is Tuning's daily readiness score calculated?

The readiness score is computed from your own SDNN samples against your personal 14-day baseline. Readings close to or above your baseline suggest your nervous system is balanced and recovering; readings well below it suggest higher stress load. Tuning combines that signal with the number of valid samples that day — a score based on few samples is shown with lower confidence, never guessed.

Does Tuning work with my Apple Watch?

Tuning reads Heart Rate Variability (SDNN) from the Apple Health app, so it works with any Apple Watch model that records HRV — including watchOS devices from Series 3 onward. Your watch collects the samples passively while Tuning turns them into insight. An iPhone is required.

Is Tuning a medical device?

No. Tuning is a general wellness app for self-regulation reference only. It does not diagnose, treat, cure, or prevent any disease, and its scores are not clinical measurements. If you have health concerns, always consult a qualified healthcare professional.

HRV Normal Range by Age: Why the Charts Don't Apply to You

Searching for a normal HRV range by age? The charts vary wildly and none of them describe you. Here is what research really shows — and the only comparison that matters.

The charts everyone finds

Type “normal HRV by age” and you will see tables with enormous ranges: 20–100 ms SDNN depending on the source. These come from population studies that average thousands of people — different ages, fitness levels, measurement methods and health statuses. They are useful for epidemiology, but they were never designed to tell you anything about your nervous system.

Why age norms vary so wildly

Age is only one of many drivers of HRV. Genetics, aerobic fitness, sleep, stress load, alcohol, medication and even the device used for measurement all shift the number. Two people of the same age can easily have baselines 50 ms apart. The idea of a single “normal” range for your age collapses under that variation.

The athlete and the office worker

A 25-year-old endurance athlete might sit at 80 ms SDNN; a sedentary 25-year-old might sit at 30 ms. Both are “normal for 25” — and both would be misled by a chart that says their value is unusual. The chart cannot see your training history, your sleep, or your baseline.

What research actually shows

Studies do find a general tendency for HRV to decline with age across large populations, and a clear link between higher HRV and better cardiorespiratory fitness. But these are statistical trends across groups — the scatter around them is huge, and the trends say nothing about where your personal number should be on a given day.

The only comparison that matters

Your nervous system has its own signature, and it changes slowly over time. That is why Tuning builds a personal 14-day rolling baseline from your own Apple Watch SDNN samples instead of comparing you to a population table. A value that looks “average” for your age can still be a meaningful shift for you — and a value that looks “normal” can still signal accumulated stress in your own context.

Key takeaway

Charts by age are interesting, not diagnostic. The honest reference point is your own baseline: are today’s readings above or below where you usually are? That question — not a table — is what tells you whether today is a rest day.