Sleep tracking
Your Apple Watch’s sleep tracking has been given an upgrade with watchOS 9, and it now includes useful data via Sleep Stages.
Sleep Stages enables you to see how much of your bedtime is spent in deep sleep, how much of it is in REM sleep (when there’s rapid eye movement, often associated with dreams) and how much of it is spent staring at the ceiling regretting things you did when you were nine. It does this by taking data from the accelerometer and heart rate sensor, and it also provides a new Comparisons tab in the iPhone app to show you your heart rate and respiratory rates over time.