Tracking Sleep Stages in iOS 16 Health App

With iOS 16 and watchOS 9, Apple introduced native sleep stage tracking that breaks down your nightly rest into Awake, REM, Core, and Deep sleep. Using machine learning algorithms powered by the Apple Watch's accelerometer and heart rate sensors, the Health app visualizes your sleep architecture through intuitive color-coded charts and detailed percentage breakdowns. This guide explains how the Health app presents each sleep stage and how to interpret your nightly sleep data.

To view your sleep stage data, open the Health app on your iPhone running iOS 16, tap the Browse tab at the bottom right, and select Sleep. The primary display shows your most recent sleep session, with options to filter your history by Day (D), Week (W), Month (M), or Six Months (6M).

The Sleep Stage Visual Chart

The core feature of the updated Sleep interface is a tiered, color-coded timeline graph. This graph plots your progression through various sleep cycles throughout the night:

  • Awake (Orange): Positioned at the very top of the graph, this represents moments when you wake up during the night, even if you do not remember them.
  • REM Sleep (Light Blue): Placed directly below the Awake tier, Rapid Eye Movement (REM) sleep indicates the phase associated with dreaming, memory consolidation, and brain restoration.
  • Core Sleep (Blue): Displayed in the middle band, Core sleep corresponds to light sleep (Stages 1 and 2 NREM). This phase typically constitutes the majority of your total sleep time.
  • Deep Sleep (Dark Blue): Located at the bottom tier, Deep sleep (slow-wave sleep) represents the most restorative phase, essential for tissue repair, immune function, and physical recovery.

Detailed Time and Percentage Breakdown

Directly beneath the interactive graph, the Health app provides a quantitative summary of your session. Tapping Show More Sleep Data reveals a detailed breakdown containing:

  • Duration per Stage: The exact hours and minutes spent in Awake, REM, Core, and Deep sleep.
  • Percentage of Total Sleep: The proportion of your overall rest attributed to each stage, helping you assess whether your sleep distribution aligns with standard healthy ranges.
  • Time Asleep vs. Time in Bed: A comparison showing how long it took to fall asleep and how efficiently you remained asleep throughout the night.

When switching from the Day view to the Week or Month view, the Health app aggregates your data into stacked bar charts showing your average time in each stage over extended periods.

Additionally, the interface integrates physiological data collected during sleep. Scrolling further down the Sleep details page displays your Heart Rate and Respiratory Rate alongside your sleep timeline, enabling you to see how your cardiovascular activity fluctuates as you transition between lighter Core sleep, active REM cycles, and Deep rest.