Actually I haven´t seen many bedrooms of married people in Japan. When I think about my parents, they sleep in the same room but in different beds. Yes, there are two single beds in their room and some space in between just like this photo.
When I was small, my parents had different rooms. My mum´s room was the back of the house and she had a double size bed while my father´s room was next to mine and at the front part of the house. He didn´t have a bed but a kotatsu where I usually watched films. In the night it turned to his sleeping place. About 10 years he slept in kotatsu.
As a child I thought my parents didn´t love each other. But later I understood that they had very different lifestyle. My mum was back then working always night shift and would wake up in the middle of the night to pick my mother up from the station where was about a half an hour drive. Then he would wake up 6:30 in the morning to wake me up and make me breakfast.
The site above says "many" Japanese couples sleep in separate rooms but what I´ve found in Japanese website is 80% of couples sleep in the same room. According to the study of the University of Toronto, about 30-40% of couple sleep separately. That´s more than Japan. In the 2013 study by the National Sleep Foundation (I´m surprised such foundation exists), only 9% of married British couples sleep in separate rooms while 28% of Japanese couple do. Still, it´s not so many.
However it seems to be true that many Japanese couples sleep in separate beds.
The couples with a small child, it´s quite common the child sleep with the parents (mostly with mum). When I was little, we lived in a one bedroom flat. We had a massive bed (for a 3-year-old) and we slept all together. I didn´t have my own room until I started the primary school and always slept with my parents.
To let the husband rest well, it also works to have separate beds and the child sleeps with the mother.
Why Japanese married couple sleep in separate beds?
This is my opinion but what I think is, Japanese people have been sleeping in futon for centuries. In Heian period (平安時代/ 794-1192) tatami was invented and since then sleeping on futon became common until Meiji revolution (明治維新/ Meiji Ishin) when beds were introduced. Then after the WII, beds became more common for normal houses.
Futon is usually made for one person. Until recent, I haven´t seen a double or queen size futon. So even a married couple, they have separate futons. Depending on the intimacy, the space between the futon can be closer or farther. You could squeeze into your partner´s futon but then a half of your body will be out of the duvet and you will get cold. In the reality, it´s not so comfortable.
One reason for Japanese couples prefer to sleep separate could be this. It´s just a custom for Japanese to sleep in the individual bed. Even in Europe, until the late 19th Century couples were sleeping separately and in the 20th Century, sleeping in the same bed has become a standard style. See? It´s not only in Japan.
Sleeping in separate rooms can be what is called Kateinai Bekkyo (家庭内別居), divorce inside the house. Although the marriage is over, for some reasons the couple keeps living together. A woman says, "We have just become flatmates, just two people living in the same house".
One day, in the classroom, a friend told me about her parents planning to get separate but had decided to live together until her brother then 15 years old to graduate high school. I understand them. Children with one parent can get teased at school in Japan. When I was at school it was a taboo to ask the classmates why they had a single parent. The parents would low their voice when they talked about the kid with a single mother. So as a child, we had learnt we shouldn´t have asked about it.
These couples, although they are practically separated, they need to live together. In this case, they own separate rooms and sleep separately. Also this has become a little more common among elder couples. My father´s generation was the generation who brought Japan as it is now. They were encouraged to work, work and work. They were married but the men have been home to communicate with their family. Then the retirement comes. The family doesn´t know what to do with this man, the father or the husband. Kateinai Bekkyo after the retirement has become common as well as the actual divorce.
There are many reasons for the couples to sleep in separate beds or rooms. But the daughter of this family says, "My parents sleep in different room, watch the same TV program and laugh at the very same moment".
You can never tell what is the happiest marriage.
Oh by the way, I´ve found there are now a family size futon. According to the size of the family, you can extend the matt by adding the piece. So now, the mother doesn´t have to squeeze herself in a single futon but all of them have enough time to sleep together.
Futon becomes bigger than the room? That´s a problem you need to solve.
More about Japanese family