I haven't seen this. I only read the description when I was purchasing another and thought, "Hmm I'll remember to buy it later" then completely forgot the title. The summary indicated that in his dream the protagonist lives his life as a woman. It may have also been vice-versa. This is not Freaky Friday, Dr Jekyll and Ms.Hyde or some obscure sci-fi 80s or older movie. From what I remember of the cover, it was at least from the 90s or 2000s to present.
I read this on the back of a DVD case, so it must be a movie not just a book. The cover was fairly plain, just an ordinary looking man and woman whom appeared to live ordinary lives (so nothing like Freaky, 2020). It seemed even boring at first glance until I read the summary. There weren't any spoilers, but what was implied was they switch lives when dreaming. Like one spirit going back and forth I guess with at least the man remembering being the woman if not both. It was a live-action movie. I'm adding also that it looked very serious. Not a comedy at all. Seemed like a drama.
The cover was diagonally divided, with the man above and the woman below. I'm pouring over movie covers right now to try to find it.
The man sleeps and wakes up as the woman. The woman sleeps and wakes up as the man. Anything else could've happened because that's about all I recall.