Sleep in a legal context: The role of the expert witness.

Sleep experts are called to assist the jury in deciding the mental state of the accused at the time of their alleged criminal behaviour. This task is difficult as the literature on many sleep disorders, particularly sleepwalking and other parasomnias, is still largely a matter of case reports and case series. The probative value of much of the evidence given is not known. Sleep behaviour in the courts present a number of difficulties which illustrate the dilemmas that face an expert witness faced with ambiguous data and uncertain principles with which to interpret them. Additionally there are substantial policy issues involved which are not always adequately addressed in expert evidence. We outline the role of expert witnesses in such cases.