Hello!
When the phone is fully charged, it is no longer accepting power until the phone drops below 100%. If the phone is in sleep mode and idle all night, it should only accept minimal levels of power. Modern smartphones are very clever with charging nowadays.
iPhones have multiple features which can extend battery lifespan in regards to overnight charging. There's "Optimized Battery Charging", which will learn your routine so that the phone will only charge to 80%, then stop, then finish charging closer to when you wake up.
Another cool feature that limits how much the phone charges at night is "Clean Energy Charging", which only accepts power when the energy from your local power grid is "clean".
You can read about these features here:
About Optimized Battery Charging on your iPhone - Apple Support
Use Clean Energy Charging on your iPhone - Apple Support
Hope this helps!