As legal defences go, it seems to have more in common with the sensational novels of Wilkie Collins than with cutting edge research - but with sleepwalking increasingly being used in court as a defence against strangulation, smothering, rape and stabbing, Edi Stark investigates how scientists are seeking to bring our knowledge about parasomnia out of the Victorian era.
The sleepwalking defence has now been used to clear about 70 people worldwide of murder, including Brian Thomas, found not guilty of murdering his wife in a caravan as they slept last year.
Thomas had a genuine sleep disorder, but there is serious concern in scientific circles that the sleepwalking defence is misused.
The brain activity occurring during parasomnia is still not fully understood, but it's thought any sleepwalker does not have control over their emotions.
Edi explores how scientists are trying to wrestle the judgement on whether an accused person is a genuine sleepwalker- or a genuine murderer - from the jury room to the laboratory.
Produced by Lucy Lloyd.
Edi Stark investigates whether the science and law are at odds in sleepwalking crimes.