If I need to disconnect from my phone in the evening but I’m talking to friends, how can I tell them I need to stop politely without lying?

"Hey listen, I'm trying to get into a better sleep cycle and am trying to go to bed at 9. Can we continue this conversation tomorrow?"

I would just be honest with them and say that you have to go. Usually when I tell my family that, they understand and are very polite about it.