By Emily Reynolds

We tend to think of sleep as a positive thing. Not enough of it, and we suffer: our moods drop, and we find it harder to both concentrate on what’s in front of us and remember what’s happened. Being well-rested, on the other hand, is associated with greater ability to communicate, to achievement at home and at work, and to superior recollection of previously learned facts or events.

Based on what we already know about the benefits of sleep on memory it may seem obvious that going to bed would also help eyewitnesses identify those they had seen perpetrate crimes. But in a new study, published in Royal Society Open Science, researchers found no such association: it was confidence, not being well-rested, that made a difference.

The team, led by David Morgan from the University of Heidelberg, asked 2,000 participants to watch a 35-second-long video clip of a man stealing a laptop from an empty office; his face was visible throughout the clip. After a distractor task, in which they were asked to unjumble anagrams of US states, half of the participants were asked to wait twelve hours before picking the perpetrator from a line-up.The other half also completed the identification twelve hours later — but only after they had gone to sleep.

Findings from previous literature suggested that this second group might perform better. But, in fact, sleep did not improve memory: participants who had gone to sleep before identifying guilty suspects were just as likely to accurately identify the correct suspect as those who had not. Instead, it was identifications made with “high confidence” that were the most accurate, regardless of whether or not participants had slept.

The two groups were also no more or less reliable at identifying suspects than a control group, which only had a five minute interval between witnessing the crime and taking the test: in each condition, participants’ confidence in their identification predicted their success.

The paper’s authors suggest that care should be taken by police to understand how confident eyewitnesses are when they identify perpetrators.

“Our research suggests police should collect expressions of confidence in the initial identification because these are predictive of accuracy regardless if there was a delay between witnessing the event or not or if sleep took place,” said senior author Professor Laura Mickes.

“Additionally, the Crown Prosecution Service (CPS) should make use of this information in legal proceedings.”

More research needs to be done here: most literature suggests that sleep does consolidate memory and improve recollection, so working out what prevents it from having a positive effect on eyewitness memory may be worthwhile. Further understanding the role of confidence could also be fruitful — and could even, as the researchers suggest, change the way we deal with crime.

– The impact of sleep on eyewitness identifications

Emily Reynolds (@rey_z) is a staff writer at BPS Research Digest