Firefighters rescued the 15-year-old girl after an astonished passer-by dialled 999. The onlooker had spotted her curled up asleep on top of a concrete counter-weight on a narrow metal beam in the early hours.

It is believed that the girl had walked unnoticed out of her home near a building site in Dulwich, southeast London. A security guard was on duty but he did not see her climb on to the crane and no one knows how she managed to do it.

Police called to the scene at 1.30am thought at first that she might be intending to throw herself off but when a firefighter went up to check on her, he discovered she was asleep. The firefighter crawled along the steel horizontal 21m-long arm of the crane and wrapped her in a safety harness.

A source involved in the rescue said: “The fireman had to be very careful because he realised the girl was asleep and he knew it might cause a serious problem if he woke her suddenly. He gently tried to wake the girl while reassuring her that she was safe and well.”

During the 2½ hour rescue, it was discovered that the girl, who was clothed, had a mobile phone in her pocket.

The source added: “The police contacted her parents to explain what was going on.

“They came down to the building site straight away. They said that she was a frequent sleepwalker.”

A London Fire Brigade official said: “It was tense for a while. One of our guys had to wait up there with her, making sure she was all right and couldn’t fall.” A specialist fire rescue team, based at Battersea, arrived at the site and used a hydraulic ladder to get her down.

The incident happened on Saturday, June 25. Police said the girl was taken to hospital as a precautionary measure but did not require treatment. Her name has not been released.

Irshaad Ebrahim, of the London Sleep Centre, was not surprised by the incident. He said: “I treat people who have driven cars, ridden horses and even attempted to fly a helicopter. However, this is one of the more unusual cases I’ve come across. Sleepwalking is nothing to do with dreaming because it occurs in a non-dreaming sleep state.”

Dr Ebrahim said that the girl could quite easily climb the crane just as she could if conscious.

Sleepwalking affects 15 per cent of children at some time (Dr Thomas Stuttaford writes). It is most common between the ages of 5 and 12 but may last into adolescence. Sleepwalkers usually have their eyes open. The action is not usually the sign of any underlying psychological problem but, if it occurs, it is likely to be worse when the child is stressed. Sleepwalking usually only lasts for a few moments but it can sometimes last for up to an hour.

The treatment is not to wake the person affected but to lead them gently back to bed. Children prone to sleepwalking should sleep in a bedroom where it would be difficult to damage themselves and that has a door and windows that can be locked.

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