Sleepwalkers and other people who have parasomnias, or involuntary behaviors they act out while sleeping, are usually harmless. But parasomnias occasionally lead to reckless actions, self-injury, and even violence against others. Enter the "sleepwalking defense," a legal argument that a criminal defendant isn't culpable because he or she acted while in a sleeplike state, without consciousness or intent to commit a crime. Here are seven historic criminal cases in which the sleepwalking defense was invoked, sometimes successfully, sometimes not.

1. Massachusetts v. Tirrell. In 1846, Albert Tirrell was acquitted in the murder of a prostitute in Boston. Tirrell slit the woman's throat, almost decapitating her. He also set fire to the brothel, then fled to New Orleans, where he was arrested. His lawyer stated that Tirrell was a chronic sleepwalker and perhaps committed the crime while asleep. The jury agreed and found him not guilty, although many contemporaries didn't buy the defendant's version of events. The Tirrell case is thought to be the first U.S. legal case in which the sleepwalking defense was mounted with success.

2. Fain v. Commonwealth. This classic case in the 1870s involved a man who fell asleep in the lobby of a Kentucky hotel. When a porter shook him to try to rouse him, the man drew a gun and shot the porter three times. While the porter held him on the floor, the man repeatedly yelled, "Hoo-wee!" He reportedly rose, left the room, and told a witness that he'd shot someone; when told who it was, the man conveyed sadness. The shooter was found guilty of manslaughter, but the conviction was reversed on appeal. Evidence that he had a lifelong history of sleepwalking and that he'd been sleep-deprived before the attack was excluded from the first trial.

3. State v. Bradley. A Texas man, Isom Bradley, testified in the 1920s that he and his mistress were preparing for bed when he became alarmed about an enemy who had made a threat against him. Fearing a secret attack, he went to bed with a pistol under his pillow. Later roused by a noise, he jumped up and fired shots. When he "found himself and got reconciled," he lit a lamp. His girlfriend was dead at the foot of the bed. Bradley was convicted of murder, but the conviction was reversed on appeal; the jury hadn't been informed of the possibility that he could have been asleep and have fired the shots without volition while in a somnambulistic state.

4. Regina v. Parks. Kenneth Parks, a young Canadian man, was acquitted in the 1987 murder of his mother-in-law after using the sleepwalking defense. On the night of the death, he arose from bed, drove 14 miles to the house of his in-laws—with whom he was said to be close—and strangled his father-in-law until the man passed out. He bludgeoned his mother-in-law with a tire iron and stabbed them both with a kitchen knife. The woman died; the man barely survived. Parks then arrived at a police station. Police said he seemed confused about what had transpired, and they noted something odd: Parks appeared oblivious to the fact that he'd severed tendons in both his hands during the attack. That obliviousness to pain, along with other factors, including a strong family history of parasomnias, led experts to testify that Parks had been sleepwalking during the attack. Not conscious, not responsible, not guilty.

5. Pennsylvania v. Ricksgers. In 1994, Michael Ricksgers was convicted of the murder of his wife. He claimed he'd accidentally killed her during a sleepwalking episode, which defense lawyers argued was provoked by a medical condition, sleep apnea. Prosecutors presented an alternative explanation: that Ricksgers was upset that his wife was planning to leave him. Ricksgers told police that he awoke to find a gun in his hand and his wife bleeding in bed beside him. He said that he might have dreamed about an intruder breaking in. That didn't sway the jury. Ricksgers was sentenced to life in prison without parole, according to the Associated Press.

6. Arizona v. Falater. In 1997, Scott Falater, a devout Mormon, stabbed his wife 44 times with a hunting knife. He then dragged her into a backyard pool and held her head under water. A neighbor who witnessed the drowning called police. Falater, who had no apparent motive, tried to mount a sleepwalking defense. He said he had a history of sleepwalking, was sleep-deprived, and was unconscious at the time of the attack. Yet Falater had tried to conceal evidence: Police found the knife, bloody clothes, boots, gloves, and undergarments stuffed in a Tupperware container hidden in his car. Also, between the stabbing and the drowning, the neighbor witnessed Falater motion for his dog to lie down, a possible sign of consciousness. A jury found Falater guilty of first-degree murder in 1999.

7. California v. Reitz. Stephen Reitz killed his lover, Eva Weinfurtner, a married woman in her 40s, during what was supposed to be a romantic Catalina Island getaway in 2001. He smashed her head with a flowerpot, leaving shards in her scalp, dislocated her arm, punctured her with a plastic fork, fractured her wrist, ribs, jaw, facial bones, and skull, and, wielding a pocketknife, left three gaping stab wounds on the back of her neck. Reitz told police that he had no recollection of the attack, though through "flashbacks" he recalled believing that he was in a scuffle with a male intruder.