“I was sleepwalking,” a public schoolboy, wearing only his boxing shorts, was heard to say after brutally attacking two fellow students and a housemaster.

Henry Roffe-Silvester, a teacher at the exclusive boarding school, was awoken in the middle of the night by footsteps coming from the dormitory directly above.

He went to investigate, and as he opened the door to the pitch-dark room, he saw a silhouetted figure who turned and struck him on the head with a hammer.

“I stumbled backwards into the corridor,” said Mr Roffe-Silvester, during his attacker’s two-month trial. “There was a second blow – I can’t remember if it was before I stumbled back – that’s a little bit hazy for me.”

He suffered six blows to the head before managing to get the weapon off the boy he now recognised as one of his students, who “slumped down” in a squat position and was heard to say: “I was dreaming.”

When paramedics arrived at Blundell’s School in Tiverton, Devon, they found “carnage” like “a scene from a horror film” with blood over the desks, the walls and the cabin-style beds.

There was no question the boy, then 16, caused the “awful injuries” to the housemaster and two sleeping dorm-mates – both boys suffered skull fractures, as well as injuries to their ribs, spleen, a punctured lung and internal bleeding.

‘Zombie apocalypse’

He told a jury at Exeter Crown Court he kept hammers by his bed for “protection” from “the zombie apocalypse” or the end of the world.

He remembered going to sleep on 8 June last year, he said, and the next thing he recalled was being in the room which was “covered in blood”.

“I knew something really bad had gone on and everyone was looking towards me,” he said.

“I didn’t remember doing anything so the only rational thing I was thinking was that I was sleepwalking.”

Prosecutors said he had armed himself with three claw hammers, then waited for his victims to fall asleep before attacking them.

But his barrister, Kerim Fuad KC, said he must have been “sleepwalking to have committed these extraordinary acts” – meaning he would be not guilty of attempted murder by reason of insanity.

The history of the sleepwalking defence

The idea that acts of violence can be committed by people who are sleepwalking isn’t new – since the 14th century, the Catholic Church recognised the idea that a sleeper shouldn’t be held responsible for killing or injuring someone.

The first English case is believed to be the Old Bailey trial of Colonel Culpeper in 1686, who was said to have shot a guardsman and his horse during a dream. He was convicted of manslaughter while insane but pardoned a few weeks later.

More incidents came to light in the Victorian era as scientists began studying the mind, among them the famous case of Simon Fraser, a known sleepwalker, from Glasgow.

Believing he was saving his family from a wild beast that had burst through the floorboards, he killed his 18-month-old son by throwing him against a wall. He was cleared but was told by the judge to sleep alone in a locked room for the rest of his life.

More recently, in 2005 Jules Lowe was found not guilty of murder by reason of insanity and made the subject of a hospital order after claiming he was sleepwalking when he beat his father to death at the family home in Walkden, Greater Manchester, after a drinking session.

Three years later, father-of-two Brian Thomas strangled his wife Christine while they were on holiday in west Wales, believing an intruder had broken into their campervan.

The nightmare was suggested to have been triggered by an earlier incident when they were disturbed by youths doing wheel spins in the car park.

Thomas was described as a “decent man and a devoted husband” by the judge after being cleared of murder when prosecutors dropped the case.

The sleepwalking defence is rare – according to sleep expert Dr Neil Stanley, who was not involved in the Blundell’s case, and tells Sky News it has been successfully used just 200 times in the English-speaking world.

Sexsomnia

But it has become increasingly common over last 30 years, says Dr John Rumbold, a lecturer at the Nottingham Law School, who tells Sky News there is a growing number of sleep experts and a greater awareness among lawyers.

In the past, reported cases more commonly involved violence, he says, but now around 80% to 90% of cases involve sexsomnia, an extreme variant of sleepwalking, that can cause people to engage in sexual activity while unconscious.

“Very often it’s drunk young men” who are accused of rape or other sex offences, he says. “They don’t really have any other defence and it’s fairly complex actions.”

Dr Stanley believes that some people who are guilty have successfully used the defence in the past and says there is a lack of knowledge of the subject among judges, lawyers and juries.

He says that in theory he could use his expertise to tell someone how to behave and what to say to convince a court they were a genuine sleepwalker.

‘Get out of jail free card’

Some see it as “a get out of jail free card”, he says, but he adds that “people, in their sleep, can kill, they can rape, they can assault – sexually or physically”.

Barrister Ramya Nagesh, who has written a book on sleepwalking and other automatism defences tells Sky News that just because it is being used more “that doesn’t mean that it’s being used in bad faith because you do have to have expert opinion”.

She thinks there should be a change in the law to allow a verdict of not guilty by virtue of a medical condition to encompass cases involving sleepwalking, epileptic fits and hypoglycaemia.

“Automatism is an outright acquittal – it feels a bit odd to say we’ll excuse them, but they might go off and do it again,” she says.

“They don’t deserve to go to prison and wouldn’t benefit from a hospital order, so it would give judges a bit more power.”

The public schoolboy, now 17, who can’t be identified because of his age, has been found guilty of three counts of attempted murder after a jury deliberated for 40 hours and he will be sentenced in October.

His relatives told the jury there was a history of sleepwalking in the family and he said his mother had found him at the bottom of a staircase in their home around a decade ago.

A ‘textbook example’?

After the attacks, the teenager told a student he was watching horror movies, while others heard him say: “I am sorry, I was dreaming.”

At his trial, sleep forensic expert, Dr Mark Pressman, who has decades of experience in the field, has seen 20,000 patients and more than 100 cases of sleepwalking violence, was called as a witness.

He described the case in court as a “textbook example”, explaining sleepwalkers could be fearful for their lives and “respond with violence to protect themselves at a very primitive level”.

“The defendant swivelled around and attacked his housemaster without knowing who he was,” he said. He was not aware he had attacked the housemaster.”

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But another expert witness, Dr John O’Reilly, told the court he did not believe the boy was asleep as a sleepwalker does not initiate violence because it is triggered by noise or touch.

Prosecutors said he had been awake shortly before the attacks, with an examination of his iPad showing he had been listening to music on Spotify, and that he had a fascination with serial killers.

‘Lucky to still be alive’

In his room, he kept a locked stash of what other pupils described as “weapons”, including shards of broken glass, screwdrivers and multiple hammers.

Police discovered he had carried out internet searches for “rampage killers”, “school massacres”, “murder with a hammer” and “killer kills while sleeping”.

He had sent alarming messages to one of his victims in the months before the attack – including a character from the horror film Texas Chainsaw Massacre wielding a hammer.

“These violent actions were repeated again and again,” said prosecutor James Dawes KC, and there was “no other explanation for his actions other than his intention to kill them”.

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Following his conviction, senior crown prosecutor Helen Phillips said the two boys were “lucky to still be alive”.

“The boy, who had a macabre interest in murder, serial killers, and violence, showed no remorse and naïvely thought that by concocting a story about sleepwalking at the time of the attack he could evade punishment,” she added.