If you want to automatically track your sleep, please enable sleep tracking on your Apple Watch. You may need to check your iPhone and Apple Watch settings to do so.
Please make sure your device’s operating system is updated to the latest version.
Please note that Apple Watch only records periods of sleep that are four hours or longer.
The Apple Watch information shared here is for your convenience. Please see Apple’s official support page for the latest information.
Apple Watch
https://support.apple.com/watch