In an old comic of the Silver Age Flash, it is discovered that a comic writer’s dreams could see into another dimension. This dimension would later be called Earth-2, and is a world where the original Golden Age Flash resided in. (A variant of this could be seen in the Justice League cartoon, but instead of the Flash we have Green Lantern instead making the connection.)
While it’s romantic to say that our dreams reflect another dimension, that’s not the only possibility. Don’t you have dreams wherein you just feel déjà vu? Or perhaps you’re dreaming of a past event, and in rarer cases, the future. Not that it helps knowing because in dreams, you kind of feel helpless. You go along with the dream, unaware that it’s a dream until you wake up. (Even worse is when you dream of a future event, and you don’t realize it in the present until it’s happened, and then you remember “I dreamed of that!” too late.) No matter how much you might try to change the past, it still goes as planned. And of course, there are times when your dreams are filled with variants: classes you never had, conversations you never experienced, meeting a friend in a different manner… it gives new meaning to the term “alternate history”.