When my 16-year-old son acquired a big bedroom at the top of the house, out of earshot of the rest of the family, and with plenty of floor space and even a battered sofabed, it seemed natural to allow him to bring home a few mates to stay over after an evening out. I prefer him to travel in numbers at night, and as long as his guests are quiet, and their parents are aware of the arrangements, I condone this practice wholeheartedly.

That is, until, among the bleary-eyed teens trooping down the staircase the next morning, I discovered a girl. “Did she sleep there with you?” I whispered fiercely. “Yeah, shared the sofa bed with Jack,” said my son, without a hint of concern.

“But, she's...a girl,” I nearly shouted. “She's a mate,” he corrected blithely, and I stumbled up the stairs behind him trying to explain why I felt there was a difference between girls and boys sleeping together “after a certain age”, and that while girls in the group were welcome to stay, they must most certainly be escorted to another room, or the sofa downstairs.

My suggestion was met with derision. “Everyone does it,” he explained slowly. “We are not children.” Except, of course, they are. Welcome to the world of co-ed sleepovers, a growing phenomenon on both sides of the Atlantic and a point of much controversy among parents, and contention between parents and teens.

“Why not let them?” says my friend Pip, who has three teenagers, one at university. “All of my kids have had sleepovers with the opposite sex present, and while they don't get much sleep, they aren't out causing trouble. They are going to be left to their own resources when they leave home, and I think that a show of trust goes a long way towards ensuring good behaviour.”

Natalie also has three children, and sees a kind of innocence in the planned sleepovers her children attend. “I don't find it a problem,” she says. They have deep, firm and trusting friendships with the same bunch of kids they've been hanging out with for years. It is a natural extension to want to spend time together in the evenings and to get away from the “get drunk” culture that sucks in so many teens. She says that there are kids from 14 to 18 in the “group”, and they happily bed down together in front of a DVD and gossip into the night. They are good at self-regulating, says Natalie, and because they are not overdisciplined, they are not hungry for excitement, and they behave responsibly to a parental show of trust.

Michelle has two children and has been regularly pestered for a co-ed sleepover by her 12-year-old daughter. Her husband, Gus, was left gasping with outrage at the suggestion. “Look,” he said. “I was a 12-year-old boy once and we were all perverts.” To which her daughter, with some disdain, responded: “Well, that was you; these boys aren't immature in that way.”

“Sharing a bed ruined my life”

But is it “filthy” to assume the worst, or a realistic appraisal of the potential problems? Sarah is 16 and still dissolves into tears at the mention of an event that took place in the Christmas break last year. She'd been at a party and had “dossed down” with a bunch of friends - most of whom she attends school with - afterwards. There were eight of them in the same room and she shared a single bed with Sean, an old friend. She had been drinking, and so had he. She ended up having her first sexual experience with him without, she says tearfully “any condoms or anything”. She was aghast the following morning, and believes it has ruined her life.

“I never wanted to have sex with someone I didn't love,” she says. “I didn't even have sex with a guy I went out with for six months. I can't talk to Sean any more. I feel really embarrassed going out because I think everyone is talking about me. I am still really worried that I might have caught a disease or that my parents will find out. It has wrecked everything.”

It's doubtful that Sarah's situation is unique. In one US survey undertaken by Teen People, a magazine for teens, 83 per cent of respondents reported that their peers were “fooling around” at co-ed sleepovers. A sleepover obviously provides an intimate opportunity for this and many kids may feel pressured to take part.

And therein lies my concern. Despite the trust we may have in our adolescents, raging hormones and intimate conditions are not an ideal mix. Add alcohol to the equation and you have a potentially toxic cocktail that could unseat even the most trustworthy of children.

Paul Perkin is the vicar of St Marks's church in Battersea, South London, and he and his wife, Christine, have written a course on parenting teenagers to grapple with these issues. Christine says: “Many parents we know seem to have little problem with a crowd of young people sleeping in the same room and think that mixing the sexes en masse creates safety in numbers, which is naive. By allowing our teenagers to sleep alongside each other we're creating an unnaturally forced intimacy that sends them the signal that parents probably aren't too worried if it goes one step farther, from sleeping alongside each other to sleeping with each other.” Her thoughts are echoed by Claire Halsey, an NHS consultant clinical psychologist, who works with families in the West Midlands and practises in Shropshire.

She says: “Preparing young people for adult life through party sleepovers is putting a potent mix of temptation, alcohol and high spirits together and waiting to see what happens. Inevitably, for some this will be disastrous as disinhibition and being borne along on the spur of the moment can lead to regret and life-changing consequences. The biggest factor for many young people, however, is often the feeling of shame afterwards, the teasing and taunting about being ‘easy' and disruption of their friendships after impulsively having sex with friends, or strangers, if the party is a large one.”

“It was completely humiliating”

It's not just the girls who are affected; boys also feel the pressure to perform according to perceived expectations. Matt, 15, attends a co-ed boarding school. On many “out” weekends, they take turns visiting friends' homes, and have a bit of a party and end up sleeping together in the same room at the end of the evening. One night, Matt found himself next to an amorous classmate and, having drunk too much, tried to oblige. He'd never had sex before, wasn't sure what was expected of him, and, after achieving an erection, couldn't work out what to do with it.

“It was completely humiliating,” he says. “I'm afraid to try again. I ended up telling my parents because I couldn't talk to anyone at school. My parents went a bit mad, and they called the other parents and said that sleepovers were fine, but the girls had to be in one room at the end of the night and the boys in another. To be honest, I feel a lot more comfortable about that.”

Tina has a 14-year-old son and she's aghast that teen co-ed sleepovers could even be considered. “It doesn't take much to realise that the potential for things to ‘go on' in such circumstances is great. I don't buy the argument that there is safety in numbers. I think that teens' experience of the media will encourage them to conclude that there is nothing wrong in making an exhibition of themselves.”

So what's wrong with saying “no” or separating them, as one parent suggests, after the DVDs or the chat has finished, and it's time to get some sleep? Hannah, a mother of three, says: “Personally I think teenagers need rules and to be told where the lines are in terms of behaviour. If there is a post-party sleepover, parents need to be present, and the girls in a different room for the night. I may be a bit of a Hitler Mummy, but I think I can live with the unpopularity.”

Karen Sullivan's sleepover solutions for worried parents

If you have a household of kids who can't get home for the night, allow them some “down” time to relax, and then shift them into separate “dorms”.

If space doesn't allow separate rooms, ask for the door to be kept open, and make it clear that you will be checking on everyone several times during the night.

If you give in to a co-ed sleepover, keep alcohol out of the mix. There's no question that it will enflame a potentially dangerous situation.

Check with other parents that they are happy for their teens to be sleeping with members of the opposite sex.

If your child says he's sleeping at a friend's, and you suspect something different, don't hesitate to call and check.

Why not say “no”? If you don't want to encourage something you disagree with, stand up and be counted. One mother I spoke to said that her own parents would have rather bricked her into a cave than allowed such a thing. A little extreme, but the message is clear. And memorable.

Claire Halsey, a clinical psychologist, suggests that parents ensure that teenagers are collected or have a practical plan to come home such as money tucked away for a taxi, bus or train, along with a time to be home.

Karen Sullivan is the author of You Want to do What? Instant Answers to Your Parenting Dilemmas (HarperCollins, £9.99)

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