If your adult child games late into the night, don't demand they stop. Ask how they feel about their routine instead. Curiosity opens the door to real conversation where you might actually learn why they're doing it, and where boundaries can come later.
Chapters
If your adult child is staying up till 3 AM playing video games, please, please do not start the conversation with, stop playing video games so late, go to sleep. And I know that's exactly what you want to say, but try something else instead. Instead of, you're wasting time playing video games, try this instead. Hey, I've noticed you've been playing video games. How are you feeling about your routine right now? And then listen. You may still need to set boundaries. You may still need to have a bigger conversation, but starting with that type of accusation almost guarantees the conversation is just going be about defending gaming. Starting with curiosity instead gives you a chance to find out what's underneath it. You cannot have a useful conversation about the next step if you never get past the argument. Save this video and try this question before the next conversation.