Apple Watch HRV: SDNN, Readings & Stress Tracking

Apple Watch HRV describes variation in the time between heartbeats. It does not measure cortisol. Cortisol+ uses available HRV with other wearable inputs to calculate a wellness score. Start with the measurement, its source and its timestamp before interpreting that score.

Where to find Apple Watch HRV

In the iPhone Health app, tap Search and look for Heart Rate Variability. Under Options, use Show All Data to inspect records and Data Sources & Access to check contributing devices. This navigation follows Apple's iOS 26 documentation; labels can differ on other versions. See Apple's Health data guide.

What does SDNN mean?

Apple's HealthKit HRV quantity is SDNN, a statistic describing variation between normal heartbeat intervals. Its unit is time, commonly displayed in milliseconds. Apple documents the SDNN data type used by apps that read Health data.

SDNN and RMSSD are different calculations. A score from another app may also use a different recording window or its own interpretation. Avoid treating two numbers as interchangeable simply because both are labeled HRV. Check the method, units and recording conditions first; our guide to HRV ranges and measurement methods explains the comparison problem.

How often does it update?

Apple describes background heart-rate readings whose timing varies with activity and an optical sensor used for HRV. That is not a fixed HRV sampling schedule. A new heart-rate value does not establish that a new HRV sample exists. Check the HRV record itself. Read Apple's monitoring explanation.

If your app shows an old value, compare it with Health before assuming the watch has stopped recording. Our HRV troubleshooting guide covers timestamps, permissions, fit and power settings.

How Cortisol+ uses HRV

HRV is one input to Cortisol+'s 0–100 wellness score, alongside available sleep, heart-rate and activity data. The model starts with defaults and can blend in personal history as readings accumulate. Missing inputs and older records affect interpretation. The published methodology describes the weights, baseline rules and limits.

The app's score is not a measured cortisol level or an observed hormone curve. We have not established its clinical accuracy against laboratory cortisol testing. It cannot tell you that a supplement worked, diagnose a hormone disorder or explain a symptom from HRV alone.

A practical way to compare readings

Record the source and timestamp, then note relevant context rather than assigning a cause to each change. Our free stress and sleep journal gives you a place to record routines and observations. A pattern in that journal is a question to investigate, not proof of cause and effect.

For app features, explore HRV stress monitoring. To compare product approaches, see our Apple Watch stress app comparison, which identifies the evidence behind its descriptions.