Didn't get your 40 winks last night? Better not get yourself arrested, or you may admit to a crime you didn't commit. False confessions are surprisingly easy to extract from people simply by keeping them awake, according to a new study of sleep deprivation. It puts hard numbers to a problem that criminal law reformers have worried about for decades.

The “crime” in question took place in a sleep lab run by Kimberly Fenn at Michigan State University in East Lansing. Together, she and Elizabeth Loftus, a psychologist at the University of California (UC), Irvine, and two of their former Ph.D. students recruited 88 Michigan State students to take part in an experiment. During two separate visits, the students worked at computers solving problems and filling out questionnaires. They were all given a stern warning: Do not press the escape key, because it will erase important study data.

After their second session, the subjects were split into two groups. Half of them were forced to stay awake all night under the watch of the researchers. Scrabble, TV shows, and a card game called euchre seemed to do the trick. The rest were allowed to get a full night's sleep. But that also required policing. "We actually had a student leave the study because he wanted to stay awake all night to study for an exam the next day," Fenn says, adding that "I certainly do not advocate this!"

The next morning, everyone received a typed statement describing their performance. The statement accused them of hitting the escape key on the first day, even though none of them actually did so—the computers recorded all keystrokes. Then they were asked to sign the statement to confirm its accuracy. If they refused, they were asked a second time to sign.

When they were asked the first time to confess, only eight of the 44 well-rested subjects admitted guilt; asking them a second time doubled that number. So even clear-headed people could be fooled into making a false confession. But losing sleep boosted those numbers significantly: 22 of the sleep-deprived subjects made a false confession when asked once, and 30 of the 44 confessed when asked again, the team reports today in the Proceedings of the National Academy of Sciences.

The study puts hard numbers to a trend on which interrogators have long relied. Sleep deprivation was used to elicit confessions in the early years of the Catholic inquisition, 17th century witch hunts, and more recently during 20th century U.S. policing and Soviet interrogations.

"Every year more and more people are released from prison who were convicted on the basis of a confession that turns out to be false," Aaron Benjamin, a psychologist at the University of Illinois, Urbana-Champaign. "It’s counterintuitive that people would admit to having committed a major crime if they hadn’t, but they do." The experiment may seem like a far cry from a real police interrogation, but "suspects are often interrogated late at night or after a long period of being held in a police station where sleep is difficult or even discouraged." And that may be leading to more false confessions.

So should all suspects and witnesses be given a full night's sleep before giving a statement to the police? Letting suspects sleep will protect the innocent but may also let more criminals off the hook by helping them resist interrogation, says John Wixted, a psychologist at UC San Diego who recently published a study on the vulnerability of eyewitnesses to false accusation. "As the authors point out, sleep deprivation would presumably increase true confessions," because it seems to compromise self-control, Wixted says. "Thus, from a purely scientific standpoint—setting aside ethical consideration—it’s a tradeoff."