Check Your Respiratory Rate
Alongside the oxygen in your blood, your Apple Watch can also keep track of your Respiratory Rate while you sleep. This is the rate at which you breathe — usually, that’s between 12 to 20 breaths per minute.
This data is recorded automatically as you sleep if you’ve set up your Apple Watch for sleep tracking and you’re wearing it to bed. You can check your respiratory rate under Browse > Respiratory in the Health app on your iPhone.