Where does my sleep data come from?

From Apple Watch through Apple Health, or directly from Oura, Whoop or Garmin. Without a wearable, the iPhone’s own sleep schedule still gives bed and wake times. Stages, heart rate dip, HRV and respiration need a wearable that measures them.

I work nights or sleep at odd hours. Does that break my score?

No. Turn on Day Mode in the app and it stops grading your sleep timing against a normal night. Your schedule is read as it is. Everything else keeps working: duration, quality and the rest of the Health Score.

Naps sync as their own sessions and show on the day next to the main night. The coach reads them too, so a late nap that pushes bedtime back gets mentioned the next morning.