People with the condition, which leaves people reclusive and incapable, are getting older and staying inside longer — a big worry for the country's government.
Hayashi Kyoko started becoming a social recluse when her high school principal started talking about university entrance exams on the first day of school.
"The fun high-school life I was looking forward to transformed into nothing more than a period of test preparation," the Japanese native told the online magazine Nippon.com.
"It was a huge shock. I'd sensed before that I didn’t belong in the strictly regimented education system. This feeling manifested itself in physical symptoms, and I stopped going to school."
And as she grew older she started working a part-time job and, facing pressure from her mother, Kyoko said she "hit her limit" and could no longer face leaving the house or meeting people.
Kyoko wasn't alone. She had become one of half a million "hikikomori," a Japanese term referring to people who avoid shut themselves at home and avoid social contact. (The term refers to both the person and the condition.)
Her lowest point was in her mid-twenties, she said: "I spent all my waking hours criticising myself... All I did was get up afternoon, eat, excrete, and breathe. I was like a living corpse. I couldn't find the tiniest bit of worth in myself. I thought my life was meaningless.
"I had this terrible kind of fury I didn't know where to direct, and I was always exhausted."
The Japanese government officially defines hikikomori as people who haven't left their homes or interacted with others for at least six months.
But hikikomori can come in various forms: One person's condition can be so severe that they lack the energy to leave their sofa to go to the toilet, like one hikikomori who spoke to the website Quartz.
Another could suffer from obsessive compulsive disorders so serious that they shower several hours a day or scrub their shower tiles for hours, such as who spoke to The New York Times. A third hikikomori said they played video games all day "as if it would tranquilise me."
Professor Jeff Kingston, an Asian studies professor at Temple University in Tokyo, told Business Insider:
"Sweeping generalisations are always misleading... [But] it seems they are mostly males who exhibit extreme symptoms of social withdrawal who often live at home with parents who take care of them.
"They rarely leave their rooms or their homes, and reportedly live in and limit interactions to the virtual world.
"It is considered a middle class malady because only hikikomori from such backgrounds can rely on the support of their families."
As of 2015, there were 541,000 hikikomori aged 15-39 in Japan, according to government statistics. There is no data on other age groups, suggesting that the figure is likely to be far larger. Some families are also loath to report hikikomori in their households, Kingston said.
Japan announced last Sunday that it would conduct its first nationwide survey of hikikomori among 40-to-59-year-olds later this year, according to the country's Kyodo news agency.
Previous surveys on the phenomenon were only of 15-to-39-year-olds, as authorities previously believed the condition was limited to young people. The government has since noticed hikikomori grow older and face longer periods of reclusiveness, Kyodo said.
The country now hopes to identify older hikikomori and understand the assistance their families need. As hikikomori grow older and their parents become too elderly to care for them, questions over their fate will become more urgent.
Kingston said: "The survey will provide more accurate information because it hasn't been done before. I suspect it will provide a basis for improving state policies towards them because it will detail their needs, but the social stigma will persist."
According to The New York Times, doctors began to observe hikikomori as a social phenomenon around the mid-1980s, when young men exhibited signs of lethargy, refused to communicate, and spent most of their time in their rooms.
There's no unifying reason why people become hikikomori. Some, like Kyoko, withdraw from society because they feel they don't know what to do with their lives and can no longer cope with the pressure from people around them. Others are triggered by events in their lives, like bad grades or heartbreak, the BBC said.
As psychiatrist Sekiguchi Hiroshi wrote on Nippon.com: "Hikikomori feel a deep sense of shame that they cannot work at a job like ordinary people. They think of themselves as worthless and unqualified for happiness. Almost all feel remorse at having betrayed their parents' expectations.
"At the same time, they are beset by internal conflict between the self that cannot go out into the world and the self that constantly condemns their failure to do so."
As Tamaki Saito, one of the country's first and leading researchers in hikikomori, told the BBC: "They are tormented in the mind. They want to go out in the world, they want to make friends or lovers, but they can't."
As hikikomori refuse to participate in society, let alone go to work, Japan's economy also suffers.
Professor Kingston said: "They diminish the size of the workforce, so contribute to a tighter labour market.
"Also, they are not self-sufficient, so when family support dries up due to death or financial problems, they will need to rely on state assistance."
Japan already faces an aging population and massive labour market shortages. There are about one and a half job vacancies per applicant in Japan, the government reported in September — the highest for more than 40 years.
According to Bloomberg, Japanese Prime Minister Shinzo Abe announced plans in late 2016 to set up counselling centres and have support staff visit hikikomori at home in a bid to boost the country's flagging workforce.
Whether the policy has worked is unclear, but Kageki Asakura, a dean at Tokyo's nonprofit Shure University, said it was "putting pressure on hikikomori."
Kyoko, the woman who was house-bound in her twenties, said she "rejoined" society around a decade later.
Along the way she almost killed herself, saw a psychiatrist, and started talking to other hikikomori. After turning 40, she also started managing hikikomori self-help groups in Yokohama, where she lives.
Other volunteer groups, such as New Start, try to get hikikomori to go to community centres, get work experience, and socialise.
New Start runs a "Rental Sister" programme, where volunteers visit hikikomori's houses and chat to them from the other side of their bedroom door to try to get them out, reported freelance photographer Maika Elan, who visited a New Start centre in Chiba-shi, a city near Tokyo, in 2016.
It usually takes a "rental sister" one to two years to coax hikikomori out their bedrooms, Elan said.
Other hikikomori set up a newspaper to shed light on the country's recluses. Established in November 2016, the Hikikomori Shimbun ("shimbun" is Japanese for "newspaper") discusses the phenomenon around the country and hopes to serve as a link between hikikomori and the outside world, according to Japan's Asahi Shimbun.
Professor Kingston said: "One can only hope that more access to various therapies and public health campaigns to destigmatise the phenomenon will encourage more to seek help, find it, and learn to manage their symptoms so that they can lead more productive and fulfilling lives."