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.

What Is Heart Rate Variability (HRV)? The 5-Minute Guide

Heart rate variability is the variation in time between your heartbeats — and a widely studied window into stress and recovery. Here is a plain-language guide to what it is and how to use it.

The beat is never metronome-regular

Heart rate variability (HRV) is simply the variation in the time between your heartbeats. Your heart does not tick like a clock — the interval between beats is constantly lengthening and shortening as your nervous system adjusts to the moment. The size of that variation is the signal.

Healthy hearts vary more

In general, a flexible nervous system produces more variation between beats; a system under sustained load produces less. Higher HRV is loosely associated with better recovery and resilience; lower HRV with accumulated stress. It is not a score to chase for its own sake — it is a readout of your nervous system’s current state.

The autonomic connection

Two branches of your autonomic nervous system control the beat: the sympathetic (“fight or flight”) speeds it up, the parasympathetic (“rest and digest”) slows it down. HRV reflects the balance between them. After a hard day you may see low variability; during sleep, when the parasympathetic branch dominates, variability naturally rises.

What it is not

HRV is not a diagnosis and not a heart-health test. It is a wellness signal — most useful for spotting trends in stress, training load and recovery. A single reading means little; a week of readings against your own baseline means a lot.

How to track it

An Apple Watch records HRV (SDNN) from your pulse throughout the day and stores it in Apple Health. Apps like Tuning then compare those samples with your personal 14-day rolling baseline, so you can see whether today is a recovery day or a push day — without comparing yourself to population averages.

Key takeaway

HRV is your nervous system’s daily report card. Learn what moves it (sleep, breathing, training load), track it against yourself, and it becomes one of the most useful health signals on your wrist.