Apple Watch directly samples motion, heart signals, location during relevant activities, ambient sound, and—on supported models—wrist temperature, electrical heart signals, and blood-oxygen-related signals. Apple then derives values such as resting heart rate, HRV, active energy, cardio-fitness estimates, sleep stages, sleep score, respiratory rate, and training load. Nutrition, cuff blood pressure, body weight, and most laboratory values generally enter Apple Health from manual entry, another app, or a connected device rather than being measured by the watch.
Editorial note
Published 2026-07-16. Last reviewed 2026-07-16. Evidence, interpretation, and Metric’s implementation are identified separately below.
1. Signals Apple Watch can sense directly
- Optical heart signal: used to calculate beats per minute during workouts, at rest, and at intervals in the background.
- Electrical heart signal: available through the ECG-related hardware and workflow on supported models and in supported regions.
- Movement: accelerometer and gyroscope signals support activity, workouts, fall-related features, and sleep estimation.
- Location, distance, pace, and elevation context: captured through relevant positioning and motion hardware during supported activities.
- Ambient sound level: supports environmental and headphone exposure features.
- Wrist temperature: available overnight on supported models after setup and baseline collection.
- Blood-oxygen-related signal: model and region availability varies; some current U.S. devices handle analysis and display differently.
2. Values Apple calculates or estimates
- Resting heart rate and walking heart-rate average from heart and movement context.
- Heart rate variability, stored by Apple Health as SDNN in milliseconds.
- Active energy, exercise minutes, stand activity, steps, distance, flights climbed, and workout summaries.
- Cardio fitness (VO2 max estimate) during eligible outdoor walking, running, or hiking conditions, using heart and motion data plus profile context.
- Sleep versus wake, estimated stages, sleep duration, sleep score, and sleep-related respiratory rate when setup and wear requirements are met.
- Overnight typical ranges in Vitals after enough nights of wear.
- Training load from recent workout intensity and duration compared with a longer workout history.
- Additional mobility and heart-related summaries supported by the user’s hardware and software.
3. Data Apple Health can hold that the watch does not directly measure
- Food, energy intake, macronutrients, micronutrients, caffeine, and water from nutrition apps or manual entry.
- Body weight, body-fat percentage, and lean mass from a connected scale, another app, or manual entry.
- Blood pressure from a compatible cuff, another app, or manual entry.
- Blood glucose from eligible external devices or apps.
- Clinical records, laboratory results, medications, symptoms, and many questionnaire-based categories.
- Workouts or sleep written by another compatible app or device.
The most important distinction
“Visible in Apple Health” describes where a record is stored. It does not identify which device measured it, whether it was manually entered, or whether it is a raw measurement or an estimate.
How to inspect the source of any value
- Open Health on iPhone and choose the relevant category and subcategory.
- Open the data or history view and inspect individual entries when needed.
- Open Data Sources & Access to see which devices and apps contribute.
- Review the source-priority order when multiple sources write the same category.
- Check units, timestamps, and whether a value was manually added.
- Confirm permissions after changing watches, phones, apps, or connected devices.
What the documentation establishes
Apple documents the heart sensor’s sampling modes, the HealthKit type identifiers, the setup conditions for sleep and Vitals, the supported workout conditions for cardio fitness, and the comparison windows for Training Load. These sources establish what a feature is designed to record or estimate. They do not guarantee a complete reading for every person or every period.
How Metric uses Apple Watch and Apple Health data
With permission, Metric reads available categories needed for its views, including workout, activity, heart, cardio-fitness, body-composition, blood-pressure, and sleep records. It can combine these with data logged in Metric, such as nutrition. Derived views—including readiness/strain and biological age—are calculated after the data reaches HealthKit; they are not new sensors on Apple Watch.
Limitations and availability
- Hardware support varies by Apple Watch model, country or region, age, setup, and paired iPhone software.
- Battery, wrist fit, skin perfusion, motion, tattoos, temperature, and workout type can affect heart-sensor availability or quality.
- Cardio fitness is an estimate from eligible submaximal outdoor activity, not a direct laboratory gas-exchange test.
- Wrist sleep staging estimates sleep from sensor patterns rather than measuring brain activity with polysomnography.
- Permissions can make a category appear empty to a third-party app even when the user has records in Apple Health.
Sources and further reading
- Apple Developer Documentation (2026): HealthKit data typesDefines HealthKit records for heart rate, resting heart rate, HRV (SDNN), sleep, workouts, nutrition, oxygen saturation, respiratory rate, and other categories.
- Apple Support (2025): Monitor your heart rate with Apple WatchExplains heart-rate sampling, resting-rate calculation, HRV measurement, optical sensing, and factors that can prevent a reliable reading.
- Apple Support (2026): Track your sleep on Apple Watch and use Sleep on iPhoneExplains sleep setup, sleep stages, sleep score inputs, historical views, and fit/battery limitations.
- Apple Support (2025): Track your overnight vitals with Apple WatchLists the overnight metrics in Vitals, the seven-night typical-range setup, influencing factors, and device limitations.
- Apple Support (2025): Track your cardio fitness levelsExplains when and how Apple Watch estimates VO2 max and the personal data and workout conditions used.
- Apple Support (2026): Track your training load on Apple WatchExplains that Apple training load compares workout intensity and duration over the latest seven days with the prior 28 days.
- Apple Support (2026): Get the most accurate measurements using your Apple WatchDescribes fit, motion, skin perfusion, tattoos, calibration, workout choice, and profile details that affect measurements.
Continue exploring
- Apple Health dashboard
- Apple Watch readiness
- Most useful longevity metrics
- Combine workouts, sleep, and nutrition
Health information disclaimer
Metric is a wellness product. This article is educational and does not provide medical advice, diagnosis, or treatment. Wearable measurements and app-generated scores are estimates; discuss symptoms, unusual readings, medication effects, and changes to a care or training plan with an appropriate healthcare professional.