As early as 1889, an Australian decision (Tolsen) noted:
"Can anyone doubt that a man who, though he might be perfectly sane, committed what would otherwise be a crime in a state of somnambulism, would be entitled to be acquitted? And why is this? Simply because he would not know what he was doing."
This criminal law defence was noted by Lord Denning said in Bratty v. Attorney-General for Northern Ireland that sleepwalking might give rise to the defence of automatism:
"No act is punishable if it is done involuntarily.... (S)ome people nowadays prefer to speak of it as automatism ... means 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 whilst suffering from concussion or whilst sleep-walking.
“It seems to me that any mental disorder which has manifested itself in violence and is prone to recur is a disease of the mind. At any rate it is the sort of disease for which a person should be detained in hospital rather than be given an unqualified acquittal.”
Quoting in part from Rabey v R, the Supreme Court of Canada used these words to define automatism in R v Parks:
"Although the word automatism made its way but lately to the legal stage, it is basic principle that absence of volition in respect of the act involved is always a defence to a crime. A defence that the act is involuntary entitles the accused to a complete and unqualified acquittal.
"That the defence of automatism exists as a middle ground between criminal responsibility and legal insanity is beyond question. Although spoken as a defence, in the sense that it is raised by the accused, the Crown always bears the burden of proving a voluntary act.
“One qualification to this statement should be noted. When the automatistic condition stems from a disease of the mind that has rendered the accused insane, then the accused is not entitled to a full acquittal, but to a verdict of insanity. The condition in that instance is referred to as insane automatism."
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