OK, since the cat's half out of the bag anyway, I just want to add a minor historical note.
Chinese women of good birth, which include people like village landowners and daughters of merchants who were technically low in social status, were raised to sleep on their backs with their hands crossed over their stomach. Why? Because they were expected to grow their hair and nails long and decorate them.
With very long hair in an era before the advent of modern haircare products, it was necessary to adopt a sleeping pose that allowed the hair to either be spread out at the head of the bed or kept in a safe bun style. That's why they had to sleep on their backs. There's another manga that actually sort of explains the same thing: Maiko-san chi no Makanai-san (Kiyo in Kyoto). There the maiko (junior geisha) have to keep their hair styled in the traditional way and sleep with the hair in that condition because it's too complicated to have to style them each morning. Upper-middle-class Chinese women did this too, depending on how many maids they had or what duties they were expected to play in the household (remember, the maids were also expected to keep their hair well-presented at all times too). On that note, the depiction of how Rifa sleeps is plain wrong. Noblewomen would have taken special measures not to sleep on their hair. My great-grandma was a very rich merchant's daughter and she used to have waist-length hair in her youth back when the Qing Dynasty was still around. To sleep properly she needed a bed that was around 30% longer than her body height. The additional length was for her hair to be spread out when she slept (she had an army of maids to style her hair every morning). You may think this excessive, but her bed was just twin XL size (203 cm long). She was only 158 cm tall and didn't need the extra space sideways because of the way she slept. Great grandma kept sleeping like this all her life even after cutting her hair to shoulder length in her 50s, and even after she'd started losing much of her hair in old age (she lived to 98). As an aside, you can actually find drawings of court women in ancient Japanese art doing the same thing but benefiting from the fact that they slept on futons laid out on tatami (thus able to make use of the whole floor space) instead of on a bed.
Similarly, especially for noblewomen who sometimes grew very long nails on their last three fingers in the Qing Dynasty, it was necessary to keep those long nails out of the way when sleeping, thus the crossed hands. Great grandma didn't have this affectation but her (last three finger) nails were always half a knuckle long until she reached her 50s.
The reason great grandma ended her traditional styles was, funnily enough, because she'd somehow managed to raise an educated, modern daughter who went on to become very active in the Indonesian independence movement and hung out with the country's founders. Great aunt ditched traditional clothes in favor of Western dress, learned to drive, bought a car, and got all the women in her family to adopt modern styles.