Heartwatch 2 for iOS has many functions for monitoring your heart rate using your Apple Watch. But it also can record and analyse your sleep data.
Now one concern may be battery drain if using this function. Actually there is nothing to get concerned about. For a start my Apple Watch manages 2-3 days on a single charge with ease. At night, I turn off wake screen with motion. Then activate sleep mode on the Heartwatch 2 app. Turning off wake screen is done using the settings app on your Apple Watch. Then open Heartwatch 2 app on your Apple Watch. Force Touch down on the screen to reveal sleep mode and press start.
Now once the screen goes off it won’t come on again while you sleep and disturb you or a partner unless you press the Digital Crown. Nor does battery life reduce that much either.
And when you wake up the data get synced across to the Heartwatch 2 app on your iPhone and you get to see how you sleep presented beautifully too.