The Automatism Defense
The automatism defense is a claim that physiological or environmental factors caused the defendant to commit criminal actions involuntarily, thus without criminal intent.
With the exception of liability without fault, which requires only actus reus, a crime requires two elements, actus reus and mens rea. While most criminal defenses attempt to excuse, justify or exculpate the defendant's criminal guilt by addressing mens rea, the automatism defense is different in that it attempts to prove that the defendant did not actually commit actus reus. Automatism can therefor apply to both conventional cases and cases of strict liability & vicarious liability. If the defendant is found to have been acting as an automaton ("a machine that moves") when the crime was committed, that is, totally unconsciously and involuntarily, then he cannot be said to have been "acting" at all, in a legal sense. And without actus reus, the defendant cannot be held criminally liable for his actions.
Automatism is a very particular circumstance, and difficult to prove in trial. Unconsciousness and involuntary action might be caused by a great many factors, including heavy intoxication, sleep, hypnosis and the like. However, automatism in the sense of a valid legal defense cannot usually hold if the unconscious, involuntary state was the result of voluntary actions. For example, a driver who falls asleep at the wheel and hits a pedestrian probably cannot successfully claim automatism, because it is presumed that he should have been aware that he was growing sleepy and pulled over to rest. He will probably be found guilty by virtue of recklessness.
The unconscious state must have been completely unforeseen and uncontrollable, as in the case of hypoglycemia, which can cause involuntary and uncontrollable movements in its victims. However, again, if the diabetic is found to have eaten irregularly or consumed alcohol, actions which are known to bring on attack of hypoglycemia, then the automatism defense will probably be denied. Likewise, if a diabetic suffers an unexpected attack of hypoglycemia while driving and is involved in an accident, the automatism defense may be denied because the diabetic should not have been driving unassisted, considering the dangerous nature of his condition.