If the idea of just staying awake for 50 hours gives you the heebie-jeebies, imagine doing so whilst on stage and trying to amuse an audience with witty improvisation.

But that’s the challenge set by the sixth annual Improvathon, which starts tomorrow at Hoxton Hall and doesn’t finish until Sunday night.

The setting is 1920s Egypt, in honour of the 90th anniversary of Howard Carter discovering Tutankhamun’s tomb, and the action starts in a hotel overlooking a Cairo market square.

From there the cast, who fly in from all over the world for this annual feast of invention, weave the story from the top of their collective heads.

And as the director, Adam Meggido has to keep tabs on the action for the more than two full days.

“I directed the whole thing last year,” he said “but I’m only the second person in the world to do it.

“We agree a setting so we can set the stage but that’s it. People pick a character, but sometimes not until an hour before we start. This year we have Salvador Dali and a young Ernset Hemingway amongst the players.

“It’s a bit like The Mummy meets Carry On Up The Nile.”

The idea for the Improvathon came from Canada – theatre maverick Ken Campbell went over to see it live and came back rather excited.

“He told us we had to check it out,” said Mr Meggido. “He said their improvisation was better than most people’s scripted stuff. So we got some of them over and started working with them and we are really delighted to be running it over here – it’s something you can’t really see anywhere else.”

The action is divided into 25 episodes – each about one hour 45 minutes long – with a recap at the start of each, so the audience can dip in without committing to the whole shebang.

There is also a special children’s episode on Sunday morning that’s suitable for the whole family.

But Mr Meggido says the crowd, who range from late night clubbers to early morning joggers, are often hooked.

“They will come for one episode to see what it’s about and then stay to see what happens. People get so involved – they put up banners, cheer and jeer. They complain if the characters they want to fall in love don’t.

“People get addicted to soap operas and this is like living in the same room as the soap. People who’ve watched the whole 50 hours call it life-changing.”

And how do the group, which includes musicians and technicians as well as those on stage, cope with the rigours of such a long show?

“We recommend people just eat and drink what they normally would. If they try to stimulate themselves they will have a crash.

“After 30 hours part of your brain shuts down because of sleep deprivation and you start performing with extreme freedom. With 25 people all in the same boat, it’s pretty special.

“Then afterwards we all go out – no one wants the night to end. We make it until about 2am, then we crash.”