I don't remember well, if ttese were 100 % calm, but you can try:
https://mydramalist.com/5785-girl-in-the-sunny-place
https://mydramalist.com/13159-pink-and-gray
https://mydramalist.com/298-winter-sonata
Winter Sonata has very pretty OST, though sometimes they argue, but I think it was in late episodes. But for sleeping I'd recommend something Japanese, they mostly don't have any music in the background.
Pink and Gray is very calm, but if you want to understand the story too, I think you would have to pay attention to it, the story is hard to understand.
Hidamari no Kanojo is wonderful movie, I think it was calm too.
Or I can recommed you some Youtube channels, that are interesting or calming.
https://www.youtube.com/channel/UC9_XmEfjzwIsXLehscbhbmw
https://www.youtube.com/channel/UCtby6rJtBGgUm-2oD_E7bzw/featured
https://www.youtube.com/channel/UCoC47do520os_4DBMEFGg4A
https://www.youtube.com/channel/UCWh7Y7qan9WNxXOucn15O3A
https://www.youtube.com/channel/UCOkV-EsKn5xHgZDLZsPnJwA