I'm always reading something, usually multiple books at a time.
On Goodreads, I belong to a group called "What's the Name of That Book?" As the name suggests, it's for people who read a book and remember details about it but not the title. They post the details they can recall and hope someone in the group will be able to produce the title.
I came across a thread called "A woman falls asleep and, in her dreams, she has a totally different life. When she sleeps in this second life, she returns to her first life, and vice versa." The original post in the thread reads:
I'm pretty sure the protagonist is a woman, but I could be wrong about that- I believe the book is from the last five years or so. The main character falls asleep and in her dream leads a second life. She falls asleep in this second life and returns to (or dreams of) the first life, and she just keeps repeating this over and over. She is extremely confused as to which life is real and which is a dream. I don't believe it involves time travel or anything like that- just two contemporary lives that are totally different.
The original poster also clarifies that the protagonist has a different name in reality A than in reality B. Person A dreams life as Person B, and in turn, Person B goes to sleep and dreams life as Person A.
One of the responses in the thread suggested The Bookseller. This didn't turn out to be the book that was being sought, but I thought the premise sounded intriguing, and I downloaded the audiobook from my library's e-collection (though I also ended up checking out the hardcover and finishing the book in print after my run today).
Unlike the book the person in the group was seeking, the dreaming only goes in one direction in The Bookseller. In the fall of 1962, Kitty Miller, a single 38-year-old woman who co-owns a bookstore, goes to sleep and dreams a reality in which it is February-March 1963, she is married to a man named Lars Andersson, has young children, and is known by her formal given name, Katharyn. Katharyn never goes to sleep and dreams Kitty's life. One blurb I came across for the book compares it to the movie Sliding Doors. Like that film, the book hinges on the idea that one small change in an incident (eight years before) could have resulted in a completely different life for the protagonist.
Both realities have their appeals and their short-comings. When Kitty is in her "Katharyn" reality, she experiences it as a weird, recurring dream, where she struggles to understand the role she is expected to play, but she also finds herself with flashes of knowledge/memories of this "Katharyn" life. In her "Kitty" life, she finds herself exploring and researching certain elements of the other reality--finding some of the same people, places, and facts she wouldn't otherwise know. Yet she keeps returning to the premise that this is all "imaginary."
Without giving away its details, I will say there is a twist toward the end, one that took me by surprise, but in a good way. I'm a sucker for alternate realities, so I recommend this book to readers who are, as well.