I call this "book mode".
What happens is I focus obsessively on the writing process. My priorities shift where I'll pass up most things other than the items at the peak of Maslow's hierarchy of needs. Working the day jobbe to provide food and shelter, parenting my child, tending my most important emotional and social connections.
Everything else sort of fades into the background. It becomes difficult to socialize casually with me. My responsiveness to email and social media drops off a lot. My pleasure reading drops close to nothing. I sleep less, take fewer showers, seek meals which require a minimum time to acquire/create/cook and to consume. I tend to be thinking the world of the story rather than the world of real life. It's a much deeper immersion than I go through when working on short fiction, for reasons which seem obvious to me.
All in all, this makes me a bit spacey, slow and goofy. And distracted. Did I mention that I'm writing a book?
So if you're a friend or someone who loves me, well, I apologize. This is part of having a writer in your life. As most people reading here already know all too well from their own experience, I'm sure.
Do you go into book mode? What does it do to and for you?