Apple Watch Vitals: understanding outliers and missing data

The five overnight measures in Vitals

When you sleep wearing Apple Watch, Vitals gathers the previous night’s data. Instead of searching through Health, you can review these five measures together.

• Heart rate: heartbeats per minute during sleep
• Respiratory rate: breaths per minute during sleep
• Wrist temperature: overnight temperature change relative to your own baseline
• Blood Oxygen: an estimate of oxygen levels in the blood
• Sleep duration: the time Apple Watch estimated you were asleep

Your typical range reflects your own recent history

After seven nights of sleeping with Apple Watch, Vitals establishes a personal typical range for each measure. It compares your own overnight records, not peers of your age or clinical reference values.

Comparison with your past is useful, but being within range does not establish health, and being outside it does not establish illness.

When you first start, there may be too little data to show the range immediately. A baseline-building period is needed.

Before searching for a cause of an outlier

An outlier tells you that night’s measurement was outside your recent range. It does not name a cause or diagnosis.

Medication, altitude, alcohol, intense exercise, insufficient sleep and changes in health may be relevant. The display alone cannot determine which caused the change.

By default, Vitals notifications arrive when at least two overnight measures are outside their typical ranges. One measure can be outside its range even when no notification arrives.

1. Check which measures were recorded

   Look for missing data and consider whether you removed the watch during the night.

2. Check how you feel on waking

   Prioritise feverishness, pain, marked fatigue or breathlessness over the numbers.

3. Review unusual circumstances the day before

   Note alcohol, exercise, travel or altitude, insufficient sleep, medication changes and other differences.

4. Review several days

   See whether values return to range the next day or several measures keep changing. Do not settle on a cause from one night.

Check missing data in this order

1. Do you have seven nights of records?

   Establishing typical ranges in Vitals requires seven nights of sleeping with Apple Watch.

2. Are sleep settings enabled?

   Check Track Sleep with Apple Watch and Sleep Focus.

3. Were charge and wear time sufficient?

   Apple recommends at least 30% charge before bed and sleeping with the watch for at least one hour. Recording stops if the battery runs out.

4. Did the watch fit your wrist?

   Wear it neither too tightly nor too loosely, with the back sensor touching your skin.

5. Are your model and region supported?

   Blood Oxygen and wrist temperature are not available on every model or in every region.

Apple Watch measurements have uncertainty

Apple Watch is a consumer device that estimates measures from wrist sensors and processing. It is not a substitute for clinical testing equipment.

A systematic review published in 2026 also reported that accuracy varies by measure, conditions and the individual’s physical characteristics.

These measures are not equally certain diagnostic results. Use them as clues to how your own records change over time.

When to seek care without waiting for numbers

One outlier does not require guessing a diagnosis. However, if changes continue for days, several measures repeatedly fall outside range or you also feel unwell, take the records to a healthcare professional.

For chest pain, severe breathlessness, fainting or altered awareness, do not wait for Vitals or the next day’s measurement. Use local emergency or medical services.