The main storyline is, every time the girl (mc) goes to sleep, she ends up in another world. This world seems to be set in olden times, and at first she thinks it's just a dream, but certain events lead to her realising it's real and that she actually went to another world. (I think what made her realise was when she saw someone she knew from her world, but I can't remember if that actually happened.)
The manga is pretty old, so I doubt this is helpful, so if you anyone knows anything similar, please let me know because it might be that one.