You wear your Apple Watch, and it populates Health app with data like Calories, Motion, Heart Rate, etc. NapBot runs Machine Learning on that data and displays your sleep analysis. Just wear your Apple Watch during sleep to track it automatically.
Please check if you gave all the needed permission to access your Health data. NapBot uses the Health app to access your calories, heart rate, motion to learn, and understand your sleep patterns. If you are not sure about permissions, please reboot both your iPhone and Apple Watch devices, open Health app -> Your Profile -> Apps -> NapBot and turn on all the permissions.
Another thing you have to do is keeping app alive in background. Please don't kill or close the app via app switcher on your iPhone.