Sinking into the almost too-hot bath, aches melting from my desk-weary muscles, I nodded politely to the women opposite. Half-familiar from the neighbourhood, we agreed that the waters were surprisingly hot, all the while submerging our bare shoulders just a little deeper.

Steam-filled and tranquil, public baths in Japan have been a haven from the stresses of daily life for more than 1,000 years. While the natural hot springs known as onsen are familiar worldwide, and can be private or public, there are also the lesser-known sento – public baths relying on regular, filtered water. Found in almost every neighbourhood and requiring complete nudity, both types of communal bathhouses have a set of strict rules on washing etiquette before entering the pristine, soap-free waters and offer a space for friends, families and even co-workers to relax and connect. These days, nearly every home in Japan has a deep-set tub perfect for a private soak and the popularity of a public dip is waning, but nowhere near as much as you might expect.

Onsen, which source mineral-rich spring water heated by natural volcanic activity, are the luxurious option for bathers in Japan. Often claiming a myriad of restorative properties, from purifying skin to easing arthritis, they are regularly cited in legends as healing animals and ancient gods. Usually designed with beautiful outdoor views, traditional cedarwood tubs and a calming atmosphere, onsen are a draw for couples and friends seeking a relaxing break from everyday life. Quiet towns like Kinosaki in Hyōgo prefecture and Kusatsu in Japan’s Kantō region have been transformed into destination resorts thanks to the promises of their water and the sense of nostalgia attached to a post-soak evening stroll in a light kimono known as a yukata.

You may also be interested in:

• Meet Japan's first female bathhouse artist

• Japan's most remote onsen

• What Japan can teach us about cleanliness

While the attraction of onsen is easy to appreciate, the everyday sento poses a more practical and seemingly obsolete option in today’s world. Originally reserved for monks during the introduction of Buddhism in 6th-Century Japan, the baths were eventually opened to the public, albeit initially only to the ill and the wealthy. The number of public sento slowly grew in the 12th Century and their use flourished in the Edo period (1603-1868) when, during this peaceful and culturally rich era, with increased economic growth, the baths became as socially vital as they were practical. Using artificially heated tap water, sento are often brighter, tiled spaces with colourful murals and lower entry prices.

Alexia Brue, an avid explorer of bathing culture around the world, delves into the history of sento in her book Cathedrals of the Flesh, comparing their past social vibrancy to that of Europe’s coffeehouses. Their importance as lively community gathering spaces is colourfully depicted in Edo- and Meiji-era ukiyo-e (woodblock prints of idyllic and playful scenes in everyday life. For example, Toyohara Kunichika, a revered 19th-Century artist and son of a bathhouse owner, created raucous scenes depicting cavorting kabuki actors scrubbing their colleagues’ backs or chatting women bathing alongside children.

Today, however, due to­­­ modern home-bathing and changing habits, the number of sento are rapidly declining: there are just 530 sento in operation in Tokyo today compared to more than 2,700 50 years ago, according to the Tokyo Sento Association. But the sento’s continued survival suggests there is something unique about the social atmosphere they provide; something that cannot be entirely replaced by cafes or bars.

Every body comes with its own imperfections and this is the only place I know where nobody cares

In my local bathhouse, a modest single-storey sento in Gotokuji, Tokyo, a soak costs a mere 460 yen (£3.25) – a rate fixed by the government in an effort to keep bathing affordable. Before sinking into the almost too-hot bath opposite my neighbours that day, I slowly shed my connections to the outside world, starting with my shoes, which I placed into a small locker at the entrance. I chatted with the owner and stepped through to the changing rooms, where I was greeted with a microcosm of Tokyo’s female population, from grandmothers to their granddaughters. Children are brought to the sento as soon as they can walk to be introduced to this rare world where nudity is neither celebrated nor shamed, simply accepted. With no swimsuits or T-shirts to cover shy bodies, here every shape and size, every scar and stretch mark is laid bare.

“Every body comes with its own imperfections and this is the only place I know where nobody cares,” said Stephanie Crohin, a French expat turned Tokyo-resident and now an ambassador for the Tokyo Sento Association.

Having fallen in love with sento while studying in Tokyo, Crohin now works in the capital and is always keen to extoll the benefits of public bathing to hesitant visitors. “Sento are not the Instagram world, but real life. [They’re] the reminder we all need when we’re constantly being crushed with the perfection of the SNS [social media] world,” she continued, highlighting the need for this reassurance for not only children, but adults too.

By offering a space for truly inclusive nudity, public baths are a unique chance to familiarise children with the natural form in a world where air-brushed, provocatively posed bodies are all too common. “Young girls learn early in life that not all women look like supermodels, and they don’t need to,” said my friend Tomomi Abiko, a Japanese native who has enjoyed onsen for as long as she can remember.

Beyond encouraging an acceptance of our own bodies and a rare chance for “skinship” – a term used in Japan for physical closeness, often between parents and children – nudity encourages a certain level of honesty between bathers. Japan is a strongly hierarchical culture that values age and experience as much in the business world as it does in the family home. A symbol of respect, the senpai and kohai relationship (similar to that of a “teacher” and “student”) can be found among school children, sports teams, colleagues and even families. Although this deference can offer great nurturing opportunities, it can also hinder honesty – with kohai not wanting to contradict or question their senpai in fear of seeming disrespectful.

When bathing, however, the seemingly simple gesture of removing your clothes serves to remove your societal position, albeit temporarily. Providing a space for commoners and the elite to bathe together, the bathhouses were a ground-breaking addition to a deeply hierarchical society.

It may seem a tad idealistic, however, to believe that removing a shirt will revoke all senses of seniority, but in Japan the concept has been taken to heart. Known as “hadaka no tsukiai”, which can roughly be translated as “naked communion”, it reflects an entirely platonic openness that can only be achieved in such an unusually close environment. Common between colleagues but also family, communal bathing allows people to broach controversial topics and speak openly and honestly about their feelings. “Come as you are; aside from the respect reserved for the elderly, everybody is on the same level: no rich or poor, no company CEO or employee,” Crohin said. “For me, in the time shared in the sento, we all share the same value.”

Young girls learn early in life that not all women look like supermodels, and they don’t need to

Since many office workers throughout Japan live in company dorms with only shared baths, communal bathing is a more common prospect than you might imagine. The baths provide a good opportunity to open up on a more personal level – be it with co-workers, close friends or family, Abiko explained. “For me, hadaka no tsukiai helps me spend quality time with my mother,” she said. “Of course, we talk at home as well, but being naked and relaxing in the hot water helps you open up a little bit more than usual and talk about things that you would not normally talk about. Probably for a similar reason, when you go to onsen with your close girlfriends, you’re very likely to end up discussing your romantic relationships.”

While these experiences come together to form an image of bathhouses as truly social places, visited by families and groups to share a connection difficult to find elsewhere, many people bathe alone too. Whether it’s a chance to unwind after work or a place to connect with nature, the bathhouses still offer something a standard apartment bath cannot. For Crohin, however, who visits many sento alone as part of her work, the community is always present: “I’m never really alone. I often talk to the ladies sharing the bath with me and the owner; sento are my family in Japan,” she said.

Mixing steam with secrets, bathhouses in Japan are a unique bubble of social space. They offer a haven from the social constructs, expectations and criticisms of the everyday world, and foster an honest but supportive environment. While their physical practicalities may no longer be required in contemporary society, there is an everlasting need for the community spirit they foster. As I sat back in my local sento, discussing favourite neighbourhood cafes with new friends, the tangible warmth captured in Kunichika’s prints seemed suddenly timeless.

Why We Are What We Are is a BBC Travel series examining the characteristics of a country and investigating whether they are true.

Join more than three million BBC Travel fans by liking us on Facebook, or follow us on Twitter and Instagram.

If you liked this story, sign up for the weekly bbc.com features newsletter called "The Essential List". A handpicked selection of stories from BBC Future, Culture, Worklife and Travel, delivered to your inbox every Friday.