"The first disrobe is the hardest."

"The first disrobe is the hardest."

"I've been nude pretty much all over Glasgow."

"My husband was appalled the first time I did it. You'd think I was working as a lap-dancer."

"I am a piece of fruit. I am a prop."

"It made me feel ready to run naked into the sea."

These are things life-models say when asked about life-modelling. Ask a model about her or his life, however, and they tell you a bunch of other stuff: about their absent father; their jealous lover; about the time they met Slash from Guns 'n' Roses and, overcome, all they could do was cry. The secret lives of life-models. Beyond the stillness, beyond the silence, there's a lot going on.

There are thought to be around 100 life-models in Glasgow. Though you get clusters wherever there are art schools - Edinburgh, Dundee, Aberdeen - Scotland's biggest city is at the centre of a boom. The Glasgow School of Art (GSA); Wasps Studios; various colleges, galleries, cafes, pubs - all run life drawing classes and none finds a shortage of willing flesh.

On a bitterly cold Tuesday, Joanne McCafferty is modelling in a building within the Botanic Gardens in the west end of the city. McCafferty is a second-year history of art student, volunteers at the Glasgow Women's Library, and hopes one day to work as a curator. She is 19 years old with long dark hair and large sad eyes. She plays bass in a band called - of course - Life Model that have been praised for their "psychedelic beauty" by the NME. Her own preferences run towards rock, and it was she who had that tearful encounter with Slash.

This drop-in class has been held weekly for the last five years. Tonight, around 25 people walked through the shivering dark, past the glowing dome of the Kibble Palace, and paid tutor Sandy Grant a tenner for a two-hour session. A real mix: men and women; twentysomethings to sixtysomethings. One woman's pregnant belly enacts a partial eclipse of her sketchpad. The room is narrow and bright, its bricks white. Two rows of plastic chairs are arched around the model.

McCafferty removes her robe and runs through a series of poses lasting from one to 20 minutes. Life-modelling has a long history, and once upon a time the duration of poses was measured by sand trickling through an hourglass. Sandy Grant, by contrast, keeps an eye on the digital display of his phone. The atmosphere of the class is hushed but engaged, not unlike an exam hall. Its sounds are the whisper of pencil on paper, the scratch of chalk, and the sighs of the heater, this last being essential during life-drawing classes in Scotland. There is nothing erotic about the experience.

"I remember telling my boyfriend I was a life-model," McCafferty says later. "It took him a while to get used to that. So I took him to a class and showed him what happens. He's completely fine about it now. Classes are completely non-sexualised. No-one's got ulterior motives."

Why would someone take all their clothes off in front of strangers? All sorts of reasons. To celebrate losing weight; to celebrate divorce; as a tribute to a sister who had it on her bucket list but died too young. And, of course, for the money. Not that anyone's getting rich, but you can make more than you would pulling pints. According to the Register of Artists' Models (RAM), the closest thing UK life-models have to a union, the going rate is £12.50 an hour, but you hear about places in Scotland that pay as low as £7.50 and as high as £15.

Rachel McCarthy, 55, is the head of RAM, has been a life-model since 1996 and once posed for the hyper realist sculptor Ron Mueck, forming one half of his work, Spooning Couple. "I think," she laughs, "I'm in the basement of the Tate Gallery."

She has 550 models on her books, right across Britain. There is, she finds, a lack of older women willing to do the job; "they think nobody could possibly want to look at their bodies and hide themselves away".

Not everyone feels that way. Anna Brewer is a 49-year-old cartoonist. She moved to Glasgow 18 months ago following 25 years living in America, latterly Pittsburgh. She loves being a life-model. "It's a shame that there aren't more older people modelling," she says. "It's wonderful to see the effect of time on a body. I have the kind of body that I really like to draw - you can see all the muscles and bones. It's like an anatomy lesson. I was fat in my 20s. I used to be about three stone heavier and just very awkward physically. Now, I feel good about myself. I've worked hard to be slim and fit."

Brewer is quite open about what she does. This would please Rachel McCarthy, who is keen to promote life-modelling as mainstream, a normal job. "I get quite political when models say, 'I don't want to use my real name,' and 'I don't want my boss to know what I'm doing.' I want to get away from this image of it being dirty. It's a proud tradition and a profession."

Society's attitude to life-modelling has fluctuated over the centuries, regarding it sometimes as glamorous and at other times as little better than prostitution. In 19th-century France, certain models, notably Victorine Meurent, who posed for Manet's masterpieces Olympia and Le Dejeuner Sur l'Herbe, became well known - or perhaps notorious might be a better word - in their own right.

Victorian Britain felt threatened by the perceived immorality of models, with Parliament debating whether funding should be withdrawn from schools employing female nudes. In 1853, an art student in Edinburgh was suspended from class for noticing one of the life-models in the street and being so forward as to say hello.

This image began to improve with the rise of the Pre-Raphaelites and the romantic idea that the dolorous beauties in their paintings, with whom they were often involved, inspired great work. This concept of the model as muse became dominant in the 20th century as the lines between artist and sitter blurred. Nina Hamnett, the "Queen of Bohemia", posed for Modigliani (later boasting that he had complimented her on having "the best tits in Europe"), danced nude in Montparnasse cafes, exhibited her own excellent paintings, and eventually - in 1956 - fell from a window of her London flat and was impaled on a fence 40 feet below. Since then, life-modelling has become increasingly professionalised, a way of earning a living, not a way of life, but a trace element of this demi-mondish allure persists in the art world's bloodstream and proves, for some, intoxicating.

Sophia Fraser is 23, a free-spirited, neurotic (she says), artistic, ambitious gamine who makes her own clothes, has no compunction about taking them off, and is "a hopeless romantic". She ascribes to herself exhibitionist tendencies, and it is hard to disagree when informed that she once made and displayed "a massive painting of my vagina" at Glasgow's Sub Club, albeit in support of a campaign to end violence against women. She is a trustee of the charity Philanthrobeats, which attempts to raise money for human rights causes through staging club nights. She is writing a dissertation about Alice In Wonderland and mathematics, plans on graduation to live and work in Berlin, enjoys nothing more than an evening in with Netflix and her cat Mouse ("He's my life"), owns "an array of kimonos" purchased by her late grandmother, and comes over altogether as part ingenue, part intellectual, part vamp, Sally Bowles meets Virginia Woolf. She wears, need one add, a raspberry beret.

Fraser has been a life-model for almost three years. Does she see herself as part of a tradition? "The Dora Maars, the Lizzie Siddals, the Edie Sedgwicks? Yeah, I do. When I went to see myself in a gallery, that's when it hit me. I do feel part of history, yeah, as a muse, I suppose. It's a really nice feeling. I like the fact I'm immortalised."

While posing, she feels a curious mix of insignificance and power. Once, modelling in the Kibble Palace, at the foot of a marble statue of Eve, she felt as beautiful, alive and human as she has ever felt.

What, though, goes through her head as she works? "My love life. I have conversations in my head with guys I like that I wish I could have in real life. You have to be in a good mental place to do life-modelling. Over the summer, I'd gone through a really nasty break-up, and having to sit and stare at a wall inside your own head for two hours was really tough. You've got to keep a straight face. You can't let anybody know your emotions. You can't crack. But I remember having to go and cry afterwards."

There are far fewer male life-models. Young men, in particular, it is thought, are more inhibited about their bodies and have concerns that any inadvertent arousal would be embarrassing and, frankly, difficult to hide.

Heathcote Ruthven, however, is not the sort to be put off by such things. A 23-year-old Londoner studying anthropology at the University of Edinburgh, he is six feet tall with a mop of hair, and his background is interesting to say the least. He was raised by his mother, a sculptor from a working-class background; his paternal grandfather is the Earl of Gowrie, a former member of the Thatcher government, published poet and friend of Francis Bacon. "Technically, I'm 'The Honourable'," says Ruthven, "but I've never really felt very honourable."

He is a "communist libertarian" with a keen interest in anarchy, for a while played percussion in a punk band, has been a political activist, participating in the mass student demonstrations of 2010, and once had his head cracked open by a police baton. He was, moreover, a double for Ron Weasley in the first Harry Potter film, resplendent in a ginger wig, although the work dried up when he grew taller faster than Rupert Grint.

Ruthven began modelling five years ago while studying drama in Cambridge, and kept it up on his subsequent return to London, having moved into a converted warehouse. His flatmates included an escort, a dominatrix and a couple who made love on webcam for £75 per hour. This made him wonder. "How different is that from life-modelling? There is definitely a sexualised element of life-modelling, but people like to ignore it."

For him, it's about income and life experience. He is fascinated by public bathing in Scandinavia, is no stranger to rolling naked in the snow, and admires the way that Nordic culture does not equate nudity and sexuality.

"Seeing someone naked is definitely not the same as having sex with them," he says, "and it seems strange that people in this country don't understand that."

Some regard life-modelling as a means of self-improvement and advancement. "I see it almost as a networking platform, a way to meet other artists," says Madeleine Virginia Brown, 23, who works four days a week at the Glasgow School of Art, sometimes for up to almost 12 hours a day. She paints, and makes collages and screenprints in her own time.

"I try to blur the lines between model and artist. I feel that I am both of those things," she says.

She discusses poses with the students of the fine art course for whom she sits, and feels she has not only a voice but an active role in the creative process. "I'm quite a shy, reserved person, but through this I've gained confidence in a number of ways," she continues. "You feel quite empowered baring all to a class full of strangers. I have control over how much of my identity I offer up. I don't have to put on a smile every day. I can kind of just be me, and I feel that it is something unique to life-modelling; there's no need to put on a front, unlike waitressing or working in a shop where you have to be overly attentive and invested in the consumer on behalf of a company.

"It can also be meditative, but quite painful, too. If I know I'm going to be doing a really long pose, I've brought in ibuprofen gel to slather over me."

Back in the Botanic Gardens, the evening is coming to an end. "OK, we'll stop there," says the tutor. Joanne McCafferty rubs at the stiff muscles of her neck and then slips her robe back on. She has gone, in that instant, from being a nude, that classical archetype, to a young woman with hopes and dreams, a taste for Finnish heavy metal, and a night-shift job in a pizza place in town.

There is something ennobling about nudity, but in a curious way it's also a sort of mask - hiding who models truly are from the artists who seek to trace their contours and shadows.

Still, McCafferty is not complaining as she enjoys a rum and Coke afterwards in the company of some of the artists who have spent the last two hours trying, with varying degrees of success, to get her form down on paper. "If this was nine to five," she says, "it would be the best job in the world." n

For information on Sandy Grant's classes, visit artclassesinthebotanics.com. For the Register of Artists' Models, see modelreg.co.uk.