Apple Watch estimates Awake, Core, Deep, and REM intervals and saves them to Apple Health. Zolt shows those intervals as Apple supplied them. It does not relabel the stages or fill in a stage that Apple did not record.
What each label means
| Label | What it means in the stage view |
|---|---|
| Awake | Apple estimated that you were awake during the recorded sleep period. |
| Core | Apple's name for the lighter stages that often make up much of the night. |
| Deep | Apple's estimate of deep, slow-wave sleep. |
| REM | Apple's estimate of rapid eye movement sleep. |
These stages naturally occur in changing cycles. A single night does not need to match a fixed stage percentage to be valid.
Where the stages come from
Apple Watch estimates sleep stages and saves them to the Health app. Zolt reads those saved intervals and presents them with the rest of your sleep information. It does not independently detect or reclassify your stages.
Apple describes these values as estimates. They are useful for reviewing patterns, but they are not equivalent to polysomnography in a sleep laboratory. Read Apple's sleep-tracking guide.
How to use the stage view
Use the view to understand the composition of the recorded night and to compare it with your own recent history.
- Start with total sleep and whether the full period was recorded.
- Look for repeated changes rather than reacting to one stage on one night.
- Compare stage changes with awake time and sleep efficiency.
- Treat a missing stage as a data question before treating it as a sleep problem.
Zolt may use the available stage data in its Sleep Score breakdown. If a stage was not supplied, the score may have less information to work with.
Keep sleep next to the rest of your data
Once your health data is connected, Zolt keeps sleep, recovery, activity, and your current plan together so you can check what changed without treating one stage as the whole story.
If stages are missing
- Open Health on your iPhone and inspect the affected night's Sleep record.
- Confirm that sleep tracking is enabled on Apple Watch.
- Check that the Watch had enough charge and was worn during the full sleep period.
- Confirm that Zolt has permission to read Sleep data.
- Review Data Sources and Access for competing sleep apps.
If Apple Health has no stage intervals, Zolt cannot reconstruct them. If Health shows complete stages but Zolt does not, contact support with the affected date and screenshots.
Medical limits
Stage estimates cannot diagnose a sleep disorder or explain persistent symptoms. If you regularly experience severe daytime sleepiness, prolonged sleep difficulty, loud snoring, gasping, or other concerning symptoms, speak with a healthcare professional.