Imagine waking up to find yourself covered in blood with no memory of what happened, only to realise that you’ve killed someone. It sounds like the plot of a crime thriller, but real-life cases of people committing violent acts while sleepwalking have come before courts around the world. In February this year, a Victorian woman who killed her husband had charges against her dropped after undergoing a sleep study which ‘may have been used to bolster her defence case’ that she was asleep when stabbing her husband to death.

So, what happens under Australian law if someone commits a crime while sleepwalking?

Sleepwalking and the law: is it a defence?

Sleepwalking – also known as somnambulism – raises an interesting legal question because it involves actions carried out in an unconscious state. Courts have long recognised that a person should not be held responsible for an act they had no control over. The defence of ‘unwilled act’ or ‘automatism’ may be raised in these circumstances. Automatism refers to actions that are entirely involuntary, meaning they were performed without conscious awareness or control.

Automatism may be regarded as either ‘sane’ (resulting in a full acquittal) or ‘insane’ (resulting in a finding that the accused is not guilty by reason of mental impairment, which is a form of qualified acquittal which may still result in detention).

In Australian courts, sleepwalking and other parasomnias have typically been treated as sane automatism, so long as they are not associated with an underlying mental illness or impairment. As put by Brereton JA (Ierace J agreeing) in a 2022 case:

‘It is… plain that in law the acts of a person who is asleep and engaging in somnambulistic activity are not willed acts; and the accused is not legally responsible for them. It makes no difference whether they would amount to murder, rape, or assault if the accused were awake and conscious.’

The difficulty in raising sleepwalking as a defence

So yes, it’s possible for a person who kills another (or does some other harmful act to them) to be acquitted if they were sleepwalking at the time. However, successfully using sleepwalking as a defence is difficult. Courts will look at several factors, including:

Medical history: Does the accused have a known sleep disorder?

Expert testimony: Sleep specialists may conduct tests to confirm whether the accused is prone to sleepwalking.

Witness accounts: Did the accused appear confused or unaware of their actions during the incident?

Behaviour before and after the act: If the person took steps to conceal the crime or fled the scene, this could suggest they were conscious of their actions.

The difficulty in establishing sleepwalking as a defence means it is rarely used, let alone successful. For example, just last week a Texas man was found guilty of murdering his twin sister and was sentenced to 15 years imprisonment after his defence team unsuccessfully argued that he was sleepwalking at the time. In that case, ABC News US reported that prosecutors said he was ‘awake and on his phone before the killing. His phone even logged his steps to and from his sister’s bedroom. He was conscious enough to turn off his school alarm and to call 911… They also believe he used a pillow to muffle his sister’s screams.’

But in cases where expert evidence supports the accused person’s claim, courts have accepted it as a defence.

Conclusion

While sleepwalking murder cases are rare, they raise significant legal and medical questions. Under Australian law, sleepwalking could allow an accused person to raise the ‘unwilled act’ or ‘automatism’ defence, even in a murder trial. But in reality, successfully raising it requires strong expert evidence and surrounding circumstances that are consistent with the accused person’s claim that they were not awake. 

Note that the content of this blog does not apply in all jurisdictions, does not constitute legal advice, and should not be relied upon. You should seek legal advice in relation to any particular matters you may have. All opinions expressed are our own, not necessarily those of any organisations with which we are connected.