Violence in the bedroom is actually quite common

Dr Peter Fenwick, Institute of Psychiatry

Dr Fenwick said US studies estimated up to four in 10 men suffering sleep disorders were aggressive, and one in seven could be seriously violent.

The phenomenon is not new. Dr Fenwick, from London's Institute of Psychiatry, told the conference the tale of a knight who stabbed his friend to death in the night, which dates back to 1600.

He told BBC News Online "Violence in the bedroom is actually quite common.

"But serious violence is, fortunately, relatively rare."

War memories

He said it was also known for sex assaults to be committed.

He added: "On one occasion, a man left his room naked, walked into another room, got into bed with a couple and put his hand on the woman's genitals."

Such sleep disorders can be treated with medications which relax the muscles and prevent movement.

Professor John Stradling, of the Oxford Centre for Sleep Medicine said these kind of sleep disorders were very rare.

He said in the many years he has run a sleep clinic, he has seen perhaps half a dozen cases.

But he said the condition could have devastating consequences: "We have seen an ex-Japanese prisoner-of-war. Every night, he dreams about still being in the prisoner-of-war camp and being beaten.

"His wife can't sleep with him. He attacks her thinking she's a Japanese guard."

He said there could be a range of causes for the condition, such as post-traumatic stress disorder or Parkinson's Disease.

But he said REM sleep disorders did not have to lead to violence: "I know of one orthopaedic surgeon who dreams he plays for Arsenal every night."