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

Abstract

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.

Keywords

Automatism expert evidence sexsomnia sleep apnoea sleep disorders

Sleep and its disorders in the criminal justice system

Sleep disorders can be the basis of a defence to various criminal offences. Medical evidence about sleep disorders can support the automatism defence, whether insane or non-insane, or simple lack of mens rea. Sleep is related to automatism in two main scenarios. The first is the defendant falling asleep during an attention-critical task (almost always driving) due to a sleep disorder such as narcolepsy or the result of sleep disruption, for example due to obstructive sleep apnoea syndrome (OSAS).

This article though will be confined to discussing the second type of sleep-related automatism, most commonly due to sleepwalking and sleep-related sexual behaviour also known as sexsomnia (although other parasomnias such as sleep terrors can cause problematic behaviour). The selection of a suitable expert is complicated by the interdisciplinary nature of sleep medicine. Sleep specialists can be respiratory physicians, neurologists, neuropsychiatrists, psychiatrists, psychologists or physiologists. There is some evidence that the discipline or specialty of the expert affects their opinion, particularly about whether sleepwalking is an insane or non-insane automatism.¹

Many expert witnesses on sleep behaviour are not psychiatrists approved under Section 12(2) of the Mental Health Act 1983. For this reason, at the trial of Brian Thomas (where C.I. was an expert witness),² a forensic psychiatrist was requested to comment on whether a hospital order would be beneficial if the special verdict of ‘not guilty by reason of insanity’ was returned.³ Interestingly, the issue of whether parasomnias are mental disorders under the Mental Health Act was not directly addressed.⁴ If this were the case, it would rule out the possibility of a hospital order due to the provisions of Section 24 of the Domestic Violence, Crime and Victims Act 2004, which states that this is not permitted unless the disorder is a mental disorder requiring specialist treatment under the Mental Health Act 1983. A recent homicide case involving an epileptic automatism did result in a hospital order (the disorder was temporal lobe epilepsy).⁵ According to Burgess, sleepwalking is an internal cause of automatism and therefore comes under the M’Naghten Rules⁶ as a disease of the mind. Therefore, the alternative disposal of outpatient supervision was arguably legally correct and indeed recommended by the two sleep experts.

The ambit of the expert witness

The expert witness’s role is to aid the jury with matters that are outside their experience. Sleep and its disorders are such an area of technical expertise. The average juror will not understand the range of complex behaviour that is compatible with parasomnia and other sleep disorders. The expert witness will offer an opinion based on the facts of the case. This may mean that a proper opinion cannot be formulated until during the trial, when the facts are fully determined. A particular difficulty for the prosecution expert witness is that they are often not permitted to speak to the main witness of the behaviour, the complainant. So, the judgement of complex behaviour may be extremely difficult prior to trial.

The expert needs to comment not just on the mental state of the accused at the time of the illegal act but also the causative factors in order for the jury to make the normative decision of whether the individual was blameworthy. If the defendant was at fault in precipitating his condition, the defence of automatism may not be available. Examples of cases where this applies include Marison and Kay v Butterworth. In Marison, the defendant would have sudden onset of hypoglycaemic episodes without warning signs. He suffered such a hypoglycaemic episode whilst driving and lost control of his vehicle.⁷ The driver of an oncoming car that he hit was killed. In Kay v Butterworth, the defendant fell asleep at the wheel and hit some American soldiers marching on the road.⁸ Both were convicted on the basis of prior fault. In the case of sleep-related crashes caused by OSAS, the main issue is prior fault. In these instances, OSAS may be a non-insane automatism, as it causes sleep, which is a normal condition (as the Court of Appeal agreed in Burgess⁶). The Law Commission discussion document⁹ outlines their concerns about people suffering from OSAS being able to argue non-insane automatism. However, there appears no mechanism for ensuring that people with a recognised medical condition cannot argue a lack of mens rea with the proposed new recognised medical condition defence. Additionally, the issue of prior fault will be relevant in cases of sleepwalking, for example sleepwalking triggered by alcohol (see discussion of Finegan v Heywood¹⁰ below).

The scientist or clinician may be unable to reject the possibility that an episode was sleep related, but the jury has to return a verdict. This uncertainty has resulted in some expert witnesses being upbraided by trial judges for being ‘vague’.¹¹ These considerations should not influence the evidence given by an expert witness, whose duty is to the court. The phenomenology of parasomnias is not well understood, and there is a need for systematic research. If the expert’s evidence is unhelpful for various reasons, then this must be stated. The reasons why a more definitive opinion cannot be given should be stated, and any necessary qualifications to the opinion should be provided, for example there may be insufficient detail about the episode.

Categorisation of sleep behaviour

Although some forms of sleep disorder are strongly correlated with neuropathology (REM sleep behaviour disorder often precedes a diagnosis of synucleinopathy by 10 years or more), other behaviours occur so commonly as to be considered part of the spectrum of normal behaviour. Nielsen reported dream enactment behaviour of erotic dreams in 16.8% of American undergraduates. This vastly outweighs the reported incidence of sexsomnia,¹² though the reported ‘behaviour’ was simply sexual arousal rather than sexual behaviour towards a bed partner.¹³

Sleepwalking is very common in childhood, with a peak prevalence of up to 40% reported.¹⁴ Confusional arousals can occur in any individual given the right circumstances. Stress and sleep deprivation are common triggers for NREM parasomnias. Several drugs have been reported to precipitate NREM parasomnias, including quetiapine,¹⁵ propanolol, lithium, valproic acid, paroxetine, amitriptyline, venlafaxine, bupropion, zolpidem and zopiclone.¹⁵ Thus, the categorisation of a parasomnia as an internal or external cause is not always straightforward because there may be a combination of a number of factors in an individual. Even in an individual known to be prone to sleepwalking, a trigger is required. Further examination of Burgess shows that the law is not as clear as simply stating that sleepwalking must be regarded as an internal cause.⁶ The Canadian Supreme Court noted that Parks was decided on different facts to Burgess.¹⁶ So the precedent of Burgess does not prevent the examination of whether a parasomnic episode had an internal or external cause on the individual facts. This is one possible explanation why juries rarely return the special verdict of not guilty by reason of insanity in sleepwalking cases. Since the case of Lowe,¹⁷ there has been only one case in the press (Fallon¹⁸). Kearns and Mackay found only two sleepwalking cases in their review of the special verdict from 1975 to 1988.¹⁹ Another explanation is that parasomnia is more often used as the basis of a defence of lack of mens rea.

The effects of alcohol

Where a defendant is a known sleepwalker but also intoxicated, it is difficult to determine whether their behaviour is due to a sleepwalking episode, alcoholic disinhibition/blackout or a combination of the two. In a large number of trials where the sleepwalking defence is argued, alcohol consumption is a factor,²⁰ particularly in cases of alleged sexsomnia (data on file). The effects of alcohol intoxication are in the realm of normal experience. The jury will be well aware that alcohol intoxication is far more common than sleepwalking, and that alcohol is clearly implicated in a far larger number of crimes than sleepwalking.²¹ So the link between alcohol and the sleepwalking defence has important policy issues, as it may offer a defence where the individual who is intoxicated might otherwise have none. Alcohol is associated with sleepwalking episodes in a minority of patients – 12% in a recent case-control study of non-forensic patients,²² which confirms the data from a qualitative interview study of expert witnesses.²³ If this minority could be reliably identified, this would help distinguish the bona fide sleepwalking defence from desperate individuals without any valid defence. However, the capacity to have sleepwalking triggered by alcohol would not be a reliable basis for inferring the episode was parasomnic rather than due to alcoholic intoxication.²⁴

The effects of alcohol on sleepwalking have been insufficiently studied. There is an anecdotal report of the effects of alcohol on sleep studies and nocturnal wandering.²⁵ This paper has been used to justify the use of the alcohol provocation test. The alcohol provocation test is problematic because it is unvalidated and its probative value is unknown. There has been much debate over its use,^17,26 and it would arguably not pass the general acceptance test.

It has been suggested that where alcohol causes a sleepwalking episode, this rules out the automatism defence on the basis of alcoholic intoxication. This interpretation arises partly from the comments in the Scottish case of Finegan v Heywood,¹⁰ where the Lord Justice-General (Rodger) stated

Approaching the matter in that way and having reviewed the relevant authorities, the court held at p 46: ‘In the law of Scotland a person who voluntarily and deliberately consumes known intoxicants, including drink or drugs, of whatever quantity, for their intoxicating effects, whether these effects are fully foreseen or not, cannot rely on the resulting intoxication as the foundation of a special defence of insanity at the time nor, indeed, can he plead diminished responsibility’. Although their Lordships were not, of course, thinking of the situation where the voluntary consumption of alcohol for its intoxicating effect induced a transitory state of parasomnia, we consider that the same approach should be applied in such a case.

However, it is the case that diseases caused by alcohol are not treated in the same way as alcohol intoxication, as the case of Harris²⁷ demonstrates (a case of delirium tremens). Lord Birkenhead LC stated that:

drunkenness is one thing and the diseases to which drunkenness leads are different things, and if a man by drunkenness brings on a state of disease which causes such a degree of madness, even for a time, as would have relieved him from responsibility if it had been caused in any other way, then he would not be criminally responsible.²⁸

The judge in Pooley did direct the jury that if his sleepwalking was purely down to alcoholic intoxication, then he had no defence.²⁹ However, the presence of alcoholic intoxication did not preclude the possibility of other excusing causes, in a parallel with Dietschmann,³⁰ where the issue was diminished responsibility. It also does not preclude other conditions related to alcohol, such as alcohol dependency syndrome as per Wood.³¹

Assessment

The assessment of the accused has two distinct aspects. The most important aspect is the assessment of the act in question. This is imprecise, even with the benefit of training and experience, and particularly when eyewitness accounts are poor quality. The expert witness may feel unable to reject the hypothesis that the accused may have been sleepwalking, but the jury has to reach a decision. If they consider that the accused was not insane, then they have to be persuaded beyond reasonable doubt. This decision may be on the grounds that the condition was non-insane automatism, in which case they need to be persuaded beyond reasonable doubt that the accused was not suffering a total loss of voluntary control. However, they may simply believe that the condition did not amount to insanity, in which case they need to be persuaded beyond reasonable doubt that the accused had the requisite mens rea as per Clarke,³² where the defendant blamed her ‘shoplifting’ on absent-mindedness due to depression. It is up to the judge whether to put the issue of insanity to the jury.

The ‘ultimate issue’ rule is now defunct, but the expert must not stray into other areas that are still the preserve of the jury. The jury is considered able to discern whether the defendant is lying, for example, although the expert witness can comment on the basis of his interview of the accused. Since there is little if any dispute that during a parasomnic episode that person is an automaton, the ultimate issue rule was particularly difficult for parasomnia cases. This syllogism would be disputed if it were accepted that there may be islands of lucidity. In any case, as Bird et al. state:

Complete lack of consciousness and control rarely accompanies a potentially criminal act; hence, difficult judgments need to be made about the degree of loss of consciousness and attention at the specific time.³³

The mental state of the accused may be apparent where he has clear memory for the events in question, clear signs of orientation to his surroundings, and purposive actions. Fragmentary memory is consistent with parasomnia. In cases where the mental state of the accused is in question, there are often plausible alternatives to parasomnia. The most common of these is alcohol intoxication and/or blackout; drug use and dissociative states are other possibilities. The possibilities are discussed in more detail elsewhere in this special issue.³⁴ Malingering is a possibility that must always be considered.

There is also the assessment of whether the person is a sleepwalker or a ‘sexsomniac’. The most important element is a corroborated history of typical sleep-related episodes. The expert witness should not support a diagnosis of a sleep disorder purely on the uncorroborated account of the defendant, as this could amount to compurgation or ‘oath-helping’. A family history of sleep disorders such as sleepwalking, sleep talking, bedwetting (nocturnal enuresis) or sleep terrors is useful support for the diagnosis of parasomnia. The role of sleep studies in confirming the diagnosis is debated. If they are used, care must be taken not to over-interpret them. One sleep expert has even stated

sleep studies are of absolutely no value whatsoever after the fact – period … sleep studies end up as in the Falater case just being a smokescreen. It just confuses everybody. And you get people arguing over things that are totally irrelevant, and the jury, how can they be expected to sort through all of this testimony about sleep studies, when in fact the sleep studies should not be allowed in the courtroom because they’re irrelevant.³⁵

However, the majority of British experts feel they have a role in corroboration. There is considerable debate over whether sexsomnia is a distinct entity. Certainly, some individuals only exhibit their sleep behaviour in reaction to the stimulus of a bed partner (which makes detection of their disorder more difficult). The diagnosis of sleepwalking and sexsomnia is clinical. Video-polysomnography (video-PSG) is neither specific enough to rule in sleepwalking or sexsomnia, nor sensitive enough to rule them out.

Sleep deprivation and induced sudden arousals by acoustic stimuli increase the diagnostic yield.³⁶ Some experts also use the technique of spectral analysis.^37,38 Neither is universally accepted as relevant in the forensic setting.³⁹ Whatever the outcome of sleep studies, they cannot address the question surrounding the behaviour on the night in question. The presence of a parasomnia is supportive of the possibility, but the specific behaviour will determine whether the particular episode was likely to be parasomnic. As Mahowald stated

[You] can prove someone is a sleepwalker … But that is only Part 1 of a two-part question. The second question is whether he was sleepwalking on the night of the murder. Only God can answer that.⁴⁰

The ‘Ambien defence’ (Ambien is a proprietary name for zolpidem) has come under much scrutiny⁴¹ (and the recent cinematic release Side Effects appears to be based on this). The effects of Ambien, like alcohol and sleepwalking, include amnesia. Dissociative states may also result in amnesia for complex behaviour during the night. Often this behaviour is also manifested during the day, and it may be apparent to experienced observers that the person is still awake. Mahowald and Schenck presented a case series of eight patients with dissociative disorder and nocturnal wandering, one of whom exhibited quite bizarre behaviour:

growling, hissing, crawling, leaping about, and biting objects in the manner of a large jungle cat for periods of up to 1 hour terminated by an abrupt collapse and unresponsiveness while perspiring profusely. Although amnestic for his actions. the next morning he invariably recalled a specific recurrent dream of being a lion or tiger let out of his cage by a woman zookeeper whom he then followed down a path.

However, when episodes occurred during EEG monitoring, this showed wakefulness throughout.⁴² These patients may require inpatient observation for diagnosis. At the present time, there may be difficulties in securing public funding for sleep studies. Sleep-related dissociative disorders may result in an acquittal just as for parasomnias, but the diagnosis is certainly relevant to disposal – the special verdict would seem appropriate for public safety.

On the other hand, driving, cooking, emailing and texting during parasomnia are all described in the literature.⁴³ It has been questioned whether sleep driving and eating are genuinely due to parasomnias.⁴⁴

Presentation of evidence

Perhaps the most crucial role of the expert is to present his findings together with some idea of their probative value. Because the expert witness’s role is to help the court, any expert evidence’s probative value ought to outweigh its prejudicial value. Where the probative value of the evidence is unknown, this evaluation is impossible.

The expert witness is there to help the jury. This role entails that their evidence has probative value, and this outweighs any prejudicial value (cf. Rule 403 Federal Rules of Evidence). This assessment is difficult with most expert evidence on forensic sleep disorders, which is highly opinion-based. There is nothing intrinsically different about the forensic setting that should exclude tests considered accurate enough for routine diagnosis. A test or technique does not have to be unerring or falsifiable to be considered helpful to the jury, especially when it is used to support expert opinion. The case of Luttrell involved the admissibility of lip reading, and it was stated, ‘[t]he fact that an expert may be wrong is no reason to deprive the jury of such assistance as may be gleaned from the evidence’. Further, it was not necessary that ‘the methods used are sufficiently explained to be tested in cross-examination and so to be verifiable or falsifiable’.⁴⁵ However, it was stated in Davie v Edinburgh Corp (No. 2):

Their duty [expert witnesses] is to furnish the Judge or jury with the necessary scientific criteria for testing the accuracy of their conclusions, so as to enable the Judge or jury to form their own independent judgment by the application of these criteria to the facts proved in evidence. The scientific opinion evidence, if intelligible, convincing and tested, becomes a factor (and often an important factor) for consideration along with the whole other evidence in the case, but the decision is for the Judge or jury. In particular the bare ipse dixit of a scientist, however eminent, upon the issue in controversy, will normally carry little weight, for it cannot be tested by cross-examination nor independently appraised, and the parties have invoked the decision of a judicial tribunal and not an oracular pronouncement by an expert.⁴⁶

There was a recommendation by the Law Commission that the judge take on an enhanced gatekeeper function, similar to Rule 702 of the Federal Rules of Evidence, which would require the judge to be an amateur scientist.⁴⁷ The reform was refused by the government, partly on grounds of costs.

An expert witness who wishes to present evidence about a questionable abnormality on a sleep study should expect to be challenged about its probative value, given the uncertainties about interpretation of PSGs. Does it really tell the jury anything about whether the episode involved was parasomnic?

An understanding of the law will affect the presentation of the evidence. The expert witness needs to know whether his evidence is required to prove (or refute) parasomnia on the balance of probabilities, or simply provide (or refute) a reasonable doubt that the person was acting voluntarily? Although the case law defines automatism as a total loss of control,⁴⁸ nonetheless states such as parasomnia where there is certainly the semblance of control have always been considered automatism (whether non-insane or insane) as per Lord Denning’s definition in Bratty v A-G for Northern Ireland:

an act which is done by the muscles without any control by the mind, such as a spasm, a reflex action, or a convulsion; or an act done by a person who is not conscious of what he is doing, such as an act done while suffering from concussion or while sleepwalking.⁴⁹

The difficulties with the law have been recognised by the Law Commission, who produced a discussion paper in 2013.⁵⁰

Policy issues

The evidence required to prove a parasomnia is a legal question, and hinges on whether the parasomnia has been judged to be an internal or external cause (where automatism is the basis of the defence rather than lack of mens rea). External causes have been characterised by examples such as ‘a blow from a stone or an attack by a swarm of bees’,⁵¹ which constituted something ‘akin to novus actus interveniens’ for Lord Goddard CJ. This implies that not all external causes would qualify. In the Canadian case of Rabey, romantic rejection was not considered something that would qualify the episode as a non-insane automatism on the grounds of a ‘psychological blow’, but rather ‘the ordinary stresses and disappointments of life which are the common lot of mankind’.⁵²

This is in contrast to the decision in R v T that the defendant’s post-traumatic stress disorder triggered by being raped three days previously was capable of amounting to non-insane automatism.⁵³ Further, it was stated explicitly in Burgess that

although sleepwalking can no doubt be triggered by external factors such as stress, such factors are merely to be regarded as external triggers of a condition the primary source of which is internal to the accused.⁶

The triggers of a particular episode were distinguished from the underlying cause.

The directions given to the jury depend on how the case is being argued. Usually, the condition is used to argue a lack of mens rea defence, although the judge can rule that the insanity defence must be considered. If the automatism defence is being argued, then it is a matter for the jury to decide whether it is insane or non-insane, unless the judge directs otherwise. The judge directed in the case of Zack Thompson⁵⁴ that any acquittal on the basis of automatism could only be on the grounds of insane automatism.⁵⁵ The expert witness might have an opinion on whether the cause of automatism is internal or external, but it is for the jury to apply the facts of the case to the law (in exactly the same way that the jury decides on the issue of insanity). Notably, the issue of risk of recurrence of the disorder, the risk of future violence, and the nature of any treatment required, are irrelevant to the question of whether the disorder should be considered an insane automatism. As Lawton LJ commented in Quick:

If an accused is shown to have done a criminal act while suffering from a ‘defect of reason from disease of the mind’, it matters not whether the condition of the mind is curable or incurable, transitory or permanent: see per Devlin J. in Reg. v Kemp [1957] 1 Q.B. 399, 407. If the condition is transitory, the Secretary of State may have a difficult problem of disposal; but what happens to those found not guilty by reason of insanity is not a matter for the courts.⁵⁶

There are various factors that may influence judicial decision making about the directions given to the jury about the insanity defence, including public interest (see Brian Thomas case),² public safety and adherence to legal principles and case law. Thus, expert opinion about the cause and future risk to the public may be crucial in determining the outcome, even though this is not the current law. The special verdict permits the imposition of sensible measures for public protection, such as the Sexual Offences Prevention Order imposed on Fallon:

banning from him from sleeping at friends' homes or any private home unless he has previously told householders he suffers from a sleep disorder and given details of his conviction [sic].¹⁸

A plain acquittal allows none of these safeguards.

Conclusion

The role of the expert witness in a case based on an alleged parasomnic episode is complicated by the law. The Law Commission’s proposals could potentially simplify the expert witness’s task, as the trigger of the parasomnia would become irrelevant, and social control assured without the stigmatising label of ‘insanity’. However, it appears that the reforms would not eliminate the possibility of elision of the new special verdict by arguing lack of mens rea. The policy issues surrounding alcohol are rightly a societal concern; any evidence about alcohol as a trigger needs to be presented very carefully. Sexsomnia is such a controversial defence that Stephen Davies had a complaint upheld that he had not been acquitted on the basis of sexsomnia⁵⁷ (although the presence of C.I. as an expert witness on sexsomnia makes it quite clear it had been part of the defence argued). The burden and standard of proof required for insane automatism compared to non-insane automatism ensures that weak medical evidence is not sufficient for an acquittal. Expert witnesses arguably have a professional duty to ensure that sleepwalking and sleep-related sexual offences are not seen as another. Twinkie defence⁵⁸ – an invention of lawyers to circumvent justice. More research is also needed on how sleep-related defences are run, and the reasons for the overwhelming preponderance of plain acquittals.

Footnotes

Funding

This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

Conflict of interest

None declared.

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