How can you tell whether someone was asleep when he committed murder?

British prosecutors withdrew charges last week against Brian Thomas, a 59-year-old retired steelworker from Wales who strangled his wife to death as she slept in their camper van. Thomas claimed he was also asleep during the attack and imagined he was fighting off an intruder. Sleep experts believed him. How did they know he was telling the truth?

They watched him sleep, for starters. To determine whether someone suffers from a sleep-arousal disorder, or parasomnia, researchers often videotape the person in an attempt to record a mid-slumber episode. They also use a polysomnograph, which monitors several bodily functions, including breathing, eye movements, and brain-wave patterns. It’s thought that homicidal somnambulism—killing someone while you’re asleep—is related to night terrors. Both usually occur in the first two hours after someone has nodded off, during deep, slow-wave sleep. Dreams and nightmares, which are far more common than night terrors, occur during the lighter, REM stage of sleep—when people tend to be silent and immobile. It’s a distinction lost on British tabloids, which dubbed Thomas “Dream Killer Dad.”

Actually observing someone sleepwalking or experiencing night terrors in a laboratory is somewhat rare. Often, the episodes are brought on by a confluence of factors, like stress and sleep deprivation. Which is why one researcher argues for depriving the accused of sleep for 36 hours to see whether this triggers the behavior. But even if you do manage to get someone to sleepwalk in a laboratory, that doesn’t prove that the person is potentially violent. To do that, researchers say, you need to provoke the sleepwalker during an episode—by touching him, for example, to see whether this prompts an aggressive response. That can be risky for all involved.

Thomas’ story made sense, given what is known about sleep violence and night terrors. He was under stress just before going to bed. Several young men—British newspapers called them “boy racers“—had been driving their cars recklessly near where Thomas and his wife had parked their van. He says that when he was strangling his wife, he imagined he was defending her from the boy racers. The strangulation happened soon after they went to sleep. Finally, Thomas had a lifelong history of sleepwalking.

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