Apple began integrating sleep-related health feature into the iPhone in iOS 13 with Bedtime, but it wasn’t until iOS 14 that more meaningful sleep-based features such as Sleep Mode came to fruition.
When Sleep Mode is active, your iPhone mutes incoming notifications with Do Not Disturb to keep you asleep and dissuades you from using your iPhone when you should be sleeping by way of a more convoluted Lock Screen that adds an additional button to tap before you can swipe up to unlock.