be in a world of (one's) own

To be consumed by one's thoughts, imagination, etc., to such a degree that one is unaware of or insensitive to one's surroundings or the outside world in general. When she starts writing, Helen is in a world of her own, and it's very difficult to get her attention. Reading is where I get to be in a world of my own, far away from the stresses and disappointments of real life. Sorry, what did you say? I was in a world of my own there for a minute.

be, live, etc. in a world of your ˈown

seem not to be aware of things happening around you; be a person who has ideas that other people think are strange: I’m not surprised he didn’t know about it — he lives in a world of his own, that boy.