At British Museum sleepovers, young historians live and breathe the past in all its gory detail. Lisa Markwell took her son on a Nordic adventure

It's an extraordinary scene. As the last straggling tourists and students drift out through the doors of the British Museum on to Bloomsbury's darkening streets, another stream of visitors is heading inside. Some are carrying backpacks, others have huge suitcases on wheels. There are supermarket carrier bags bulging with crisp packets, and wicker baskets from upmarket shops, crisp linen pillows and a few cuddly toys tucked under arms...

The British Museum, the most staid and sensible of our grand old institutions, is holding a Sleepover. These events, held since 1996, happen several times a year and have become a much-prized hot ticket for children with a keen sense of adventure and a bit of a hunger for history. It's part of the Museum's Young Friends scheme, a £20-a-year deal for eight to 15 year olds who sign up for sleepovers, visits behind the scenes, talks, art workshops and trips to historically interesting sites.

If this all sounds incredibly worthy and boring, the kind of gift a godchild accepts with barely disguised disgust, get with the programme. History is cool. In fact, history is, to use the argot currently favoured in Primary Year 3, "heavy". This is borne out by the massive success of the Horrible History series of books (Awesome Egyptians, Dark Knights and Dingy Castles, and the latest title Villainous Victorians), whose irreverent tone plays very well with children.

For my son, a nearly-eight-year-old boy with a Viking project recently handed in to his teacher and a love of all things gory, the Sleepover is the most eagerly anticipated date in his diary (well, after Christmas and his birthday). Peter has already been to an Egyptian-themed sleepover with his dad. He's in a state of heightened anticipation.

A couple of days ahead of the event, the Young Friend co-ordinator Rebecca Murdoch sends through an itinerary - and, my goodness, we're going to be busy. With two other mums taking their daughters, we plan a strategy. I buy inflatable mattresses and a foot pump from Argos (forget sleeping rough like a real Viking; those marble museum floors are pretty unforgiving), while the others are in charge of midnight feasts, sleeping bags and transport.

The line for registration snakes around the Great Court, the glassed-over centrepiece of the British Museum. I notice many parents who are old hands at this, with minimal luggage and wearing very comfortable clothes. We, meanwhile, have about the same amount of bags as for a fortnight's holiday, including a giant cool box (although I resisted smuggling in contraband white wine in an apple juice carton).

The 200-plus guests (tickets for the Sleepovers always sell out) are divided into four groups, identified by coloured sashes. Then there's an unseemly scramble to find somewhere quiet to lay out sleeping mats and make camp. Being late and inexperienced, we stroll up to find all the best spots are taken and are forced to pitch down in the middle of the Egyptian Sculpture Gallery, between a giant scarab beetle and an even bigger mummy's tomb. We pass a family who've clearly been here before - mum and dad have a self-inflating double bed tucked discreetly behind a large exhibit.

As keen youngsters walk as fast as they can to the preliminary safety lecture (first rule: no running), old hands compare notes: "Where are you?" "Oh, under the Rosetta Stone..." This theme continues with the introductions to the Young Friends ushers and a run through the rules. To widespread titters, two Sleepover veterans admit that the rules include, "No jumping about with a sleeping bag on your head" (apparently there was a nasty collision with a stone exhibit last year) and "no children near the tea and coffee urns". They'll be lucky - it's only 7pm and I already need some strong caffeine to keep going.

The first activity is a rip-roarer - the re-enactment of a battle between plucky Anglo-Saxons and marauding Vikings on the front lawn of the museum. What passing restaurant-goers must think is anyone's guess, as hoardes of children scream encouragement at axe-wielding actors, including "Dagmar, thane of the Saxons", a convincingly terrifying leader. The temperature drops and at 8.30pm the purple group go into the Great Court to start work on constructing two Viking longboats.

The beautiful stone floors are safely covered and the tables sheathed in newspaper in preparation of 50 children liberally applying paint to dragon figureheads, masts and the inside of loo rolls (intended use unknown). Parents divide into those who are simply accompanying their offspring for safety, who take up positions near the urns, and those who are a bit, well, pushy, offering encouragement and instructions: "Da-arling, would that shield really be purple with orange spots, hmmm?"

After a welcome pitstop for food and subtle jostling for a better sleeping position amongst the pharaohs, the next activity is Nordic Tales in one of the Viking galleries. In previous years the museum has been chilly (warm clothes to sleep in advised), but maybe Rebecca Murdoch has had a word with the engineers because tonight it's toasty warm. The small folk, now in pyjamas or Young Friends T-shirts, look for all the world like seasoned gig-goers. The actor telling his Nordic tale, however, is actually melting in his fur cloak, wool stockings and layers of tweedy smocks. It doesn't prevent him from keeping listeners agog as he describes battles, beheadings and other gory stories. When he bellows, "I will not sully my sword on this piece of scum. I spit on you," boys half rise to their feet. When he brings the head of his slain "brother" out of his knapsack, there are a few anxious looks to mums (wonder how many nightmares that evokes later...).

At 11pm, our ushers take us into a cooler basement room to carve Viking runes on to tablets of polystyrene, but to be honest, many in our group are flagging. We carve our names with a mixture of pride and exhaustion and slip away to the loos before the midnight rush. Safely bedded down by 12 o'clock, most of the younger Friends are quick to nod off, but there are whispers and torch flashing well past lights-out.

It's this part of the activity that is most surreal. I'm lying in a cavernous gallery, surrounded by strangers, watched over by stony-faced Egyptians. My son is dreaming about thanes and shields and chain-mail vests. Finally, after much shushing, even the most excitable child falls asleep.

Once more it's day - 6.50am to be precise - and the queue starts forming for breakfast. Weary parents who didn't bring enough bedding try to summon up the strength for the final activity - looking around the Living History Village, in our case. But because of heavy rain, the village has moved indoors. A re-enactment enthusiast talks his rapt audience through amputations and cauterisations, Viking style. Peter has his arm "cut off", much to his delight. Other children learn about armour, weaving and calligraphy.

The whole tiring but exhilarating event ends with a parade around the museum with the completed longboats, and prize-giving, introduced by Sandi Toksvig, the patron of the Young Friends. It's inspiring to see so many pre-teens getting into learning in such an imaginative way. And the cost, £25 each, is worth every penny to parents looking for ways to stimulate their offspring; you'd pay a babysitter that on a weekend. It's a precious Saturday night given up by parents, but there can be fewer more worthy causes. So my heart only sinks a little bit when Peter asks if he can come back in July for the next Egyptian night.

For more information, contact the Young Friends of the British Museum (020-7323 8605)