How does "Sign in with Apple" work?

You can create a 7Sleep profile by signing in with your Apple ID. To do this, tap the "Sign In with Apple" button in the app during the registration process, check your information, and log in quickly and securely with Face ID, Touch ID, or the code of your device.

You can find more information here:
How to use Sign in with Apple
Manage the apps you use with Sign in with Apple

Requirements to use "Sign in with Apple":
• You must use an Apple ID with two-factor authentication.
• You must be logged into iCloud using this Apple ID on your Apple device.
"Sign in with Apple" is available on Apple devices running iOS 13 or later and iPadOS 13 or later.

Not sure which email address was used when you registered via Apple ID?
It is either your Apple ID email address or a randomized anonymous email address generated by Apple if you have opted not to share your data with 7Sleep. You can view your 7Sleep profile email address at any time in your profile in 7Mind Sleep.