iOS 13:Go into Apple Health app. It has a white icon with a red heart. Tap Browse tab twice. Tap Heart.Tap Heart Rate.Scroll all the way to the bottom. Select Show all data.Select last night. This will now show all the heart rate readings that the watch has captured for the day. Scroll to the period that you were asleep. You should see heart rate readings every 2-10 minutes. You will find that some readings are missing.