If you have an apple watch
Our recommendation is to wear your watch at night and track your sleep with Apple Health, which syncs to Gyroscope automatically. More importantly, it provides the resting heart rate and HRV measurements which are used by Health Score.
For the simplest sleep tracking is to use the free sleep++ app on your phone. In settings, enable the Apple Health sync — and then it will use your watch at night to automatically save the sleep to apple health. You will automatically see it in Gyroscope each morning without needing to do anything!
With iOS14 and the new Watch OS, you can also track sleep natively with Apple and not need any 3rd party apps.
If you don't have a watch or other wearables
The sleep AI is more intended for people with no watch or any other sleep data, and in that case you can access it by opening the daily report for any day — and then it will show a suggested sleep that can be saved.