The women baring all in Smoke Sauna Sisterhood film

Female friends have long gathered together to share life stories and laughs, but they generally keep their clothes on.

In the documentary film Smoke Sauna Sisterhood, however, the women are naked.

Shot over seven years in Estonia, the film sees a group of women strip off for a four-hour session of sweating and soul-bearing, before plunging into an icy lake.

The camera is remarkably close-up, You see every bead of sweat and fold of skin, as the women divulge their innermost thoughts and memories.

They are all completely at ease with each other.

We see them rub salt into their skin, slap each other on the back with leafy twigs and pour water onto scalding bodies, in the heat of the sauna.

In summer, we also see the women squatting not far from each other to go to the toilet in the woods, while a dog runs between them.

Some women's faces can be seen during filming, while others preferred to remain anonymous.

The camera lens remains remarkably clear in the depths of the sauna, given the room is full of smoke and steam.

"It's a safe space where all your emotions can be shared and heard," director Anna Hints says about her film, which won a directing award at January's Sundance Film Festival.

She learned to love the ritual of the hot, smoky log cabin as a child, growing up in eastern Estonia.

It is something to be treasured, having been added to Unesco's list of the Intangible Cultural Heritage of Humanity.

For Hints, the sauna is about the deep emotional connection between women, and the release of toxins - both physical and emotional.

She recalls the moment when she became hooked on the age-old tradition, after witnessing her grandmother purging some painful emotions.

"There was one very significant moment is when I was 11," she says.

She and the female members of her family had gathered to have a sauna together.

"My grandfather had died and his body was in the house. It was one day before funeral.

"And there, in the sauna, my granny revealed my grandfather had actually betrayed her with another woman.

"She said how difficult it was... and she released all the emotions connected to that. It took hours.

"And then we went out - and there was peace."

This is what we see over and over in the film, as the seasons change and the women keep returning to their "safe space".

They talk about their relationships with their mothers, laugh about sex, and discuss how they feel about their bodies.

One woman talks about the moment she came out as gay to her parents, while another expresses how it felt growing up in a culture that favoured boys.

When something distressing comes up - there are some no holds-barred conversations about sexual assault and cancer - the women listen intently, absorbing all that is said.

Emotions pour out, like the constantly flowing water of the sauna.

The women sing and repeatedly chant their support: "Take the pain away,".

Then we see them plunge afterwards into the icy pool dug into a frozen lake outside.

It looks painful as they pad over the ice, naked except for socks, but they all look rejuvenated afterwards.

"All of us have stories inside," Hints says. "As you sweat, first there is more [on the] surface and then there's dirt coming out - and it's the same with emotions."

She talks about how "all of us have those stories and deeper layers of experiences".

"Something magical happens when you're there, naked in the darkness, with this community around you. And when you share something, nobody starts to teach.

"When you share, another listens, and that is a huge power. That is part of the healing itself."

Hints also offers the audience at her film screening in Sheffield a new experience, just before it begins.

She stands at the front and sings a traditional welcome, inviting us to join in: "All your fears in cold water, All your shame in cold water, All your pain in cold water."

This doesn't spark a huge singalong, but it does get a warm round of applause, and helps give an indication of the film to follow.

Hints explains that in order to shoot the footage, the technical equipment had to be acclimatised.

"It's really a challenge," she explains.

"The average temperature was 80 degrees, and the average session lasts four hours, and then you go out and then you'll come back in."

The camera lens had to be prepared in advance, by being put on the floor of the sauna two hours before filming started, to adjust to the heat.

There was also a second lens outside, as the camera inside the sauna could not be used outdoors due to temperature difference.

She says they had two lenses break during their seven years filming the documentary, which is perhaps not surprising.

"I had to, as director, not just hold the space in emotional terms, but also physically, checking that everyone is hydrated and we don't wait too long to go out," she adds.

With the camera's focus so firmly on the women, Hints talks about how important it was for her how their bodies were filmed.

She had a male cinematographer, and was at pains to avoid the film portraying the women in a sexualised way.

"So he's very sensitive, but in the beginning, when we were finding the language, there was a male gaze, and he was not aware of that," she says.

"It became a growth journey, of being aware that actually he is used to watching female bodies in a certain way.

"And then adjusting that, and starting to look beyond desire or judgement."

Gayle Sequeira wrote in Film Companion that Hints achieved her goal, saying: "The gaze isn't sexual, but empathetic. These bodies are the maps on which the history of these women's lives has been written."

Variety's Jessica Kiang added that "the small, smoky, steamy miracle of this film is how it creates something so intangible, so lyrical, from the absolutely elemental: Fire, wood, water and lots of naked female flesh".

Vox's Alissa Wilkinson called the film, which has a 100% rating on film review aggregator Rotten Tomatoes, a "gorgeously captured space, carved out away from the world of men".

Hints ends by telling another anecdote, about a woman who learned to feel positive about her body having seen the film.

"She wrote to me afterwards that she booked a photographer," she says.

"She had been so angry about her body throughout her life, she hated her body.

"She's just gonna do some beautiful pictures and accept her body the way it is.