Sleep does not improve memory in eyewitness identifications

By Dr. Ananya Mandal, MDDec 4 2019

A new study from British researchers has revealed that sleep does not improve memory in an eyewitness. The study titled, “The impact of sleep on eyewitness identifications,” was published in the latest issue of the journal Royal Society Open Science on the 4th of December 2019.

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Lead author David Morgan at Royal Holloway, University of London, was aided by others including Professor Laura Mickes, senior author in the School of Psychological Science, at the University of Bristol and other colleagues from the Universities of California, USA and Birmingham. The team explained the sleep has been known to improve memory functions. They attempted this study in order to check if sleep could enhance memories among eye witnesses when it came to identification of suspects from line ups. The team wrote, “Evidence strongly indicates that sleep yields substantial improvements on recognition memory tasks relative to an equivalent period of wake.” They add that there have been no studies that explore this area. They wrote, “Eyewitnesses to crimes are often presented with a line-up (which is a type of recognition memory test) that contains the suspect (who is innocent or guilty) and fillers (who are known to be innocent).” Results of the study revealed that sleep may “enhance the ability to identify the guilty suspect and not identify the innocent suspect (i.e. discriminability). Sleep may also impact reliability (i.e. the likelihood that the identified suspect is guilty).”

The team explained the concept of line up in their article stating, “A line-up is a type of recognition memory test that helps police assess the likelihood that the suspect is indeed the offender. Line-up procedures generally involve presenting the eyewitness with the police suspect among fillers. Fillers are known to be innocent, and physically resemble the suspect or match the description of the offender.” The authors continue, “Of the three possible responses, an eyewitness can identify the suspect, identify a filler, or identify no one; whether the suspect was identified is typically of most applied interest. In this regard, if the suspect in the line-up is guilty, and the eyewitness chooses him or her, then that is a correct identification, and if the line-up is rejected, then that is a miss. If, on the other hand, the suspect in the line-up is innocent, and the eyewitness chooses him or her, then that is a false identification, and if the line-up is rejected, then that is a correct rejection. To put another way, two outcomes are errors (misses and false identifications), and two outcomes are accurate (correct identifications and correct rejections).” The importance of correct identification and eye witness reliability cannot be overemphasized. The team wrote, “One type of analysis assesses discriminability, which is the ability for eyewitnesses to distinguish innocent from guilty suspects. Procedures that reduce false identifications and increase correct identifications are procedures that have better discriminability than those that do not.”

For this study thus the team included 4,000 participants who took part in an online study. The participants were shown a video of a mock crime and they had to make a correct identification from a line up of fake suspects and also rate their level of confidence in the identification. They made 2,000 of the participants sleep overnight between the video and the line up or remain awake during the day between the video and the line up for identification. The other 2,000 participants served as control for the first group. These 2000 participants were shown the video and tested right after it with the line up for identification. Results showed that those without sleep in between and those with 12 hours between the video and the line up were equally good at identification of the suspects.

The team wrote that their video was a 35 second clip showing a “young adult white male (the target) stealing a laptop and mobile phone from an empty office”. All the while the suspect’s face is clear. After this 6 photographs were lined up for identification.
The authors could conclude, “The prediction that participants in the sleep condition would have greater discriminability compared to participants in the wake condition was not supported.” Explaining this Professor Laura Mickes said, “Based on prior work that sleep improves memory, we predicted that it would in this case too. It did not. There was no difference in memory performance between the sleep and wake groups. There are two major implications from our research: sleep does not benefit eyewitness identification, nor does it impact reliability. The fact that reliability is not impacted by various factors - sleep vs. wake; short vs. long retention intervals - is in line with a growing body of evidence that eyewitness reliability holds across a host of factors.”

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

The study reveals that sleep did not improve the memory of the eye witnesses and the participants who spent time sleeping were similar to those who remained awake between the video and the line up. Eye witness reliability (ability to correctly identify the guilty and the innocent) also did not change with sleep. Further the gap of 12 hours (sleeping or awake) and 5 minutes between the video and the line up did not affect the reliability of the eye witness found the study.

Authors wrote in conclusion, “Sleep joins the growing list of variables that do not impact experimental eyewitnesses' reliability. It also did not affect discriminability. Thus, the widely touted benefits of sleep were not found in a large-scale forensically relevant eyewitness identification experiment.” They wrote in addition, “Further well-powered conceptual and direct replications should clarify the benefits of sleep on recognition memory.”

The study was funded by the Economic and Social Research Council (ESRC).