Can Sleepiness Affect Your Eyewitness Memory?
It’s a given that as college students, we all feel tired from time to time. Well, maybe more than from time to time. Walking across campus, have you ever heard people saying things like “I got two hours of sleep last night,” “I slept terribly last night,” “I’m going to pass out right now,” or something along those lines? I’m sure you have at some point. 
We have all heard that it’s important to get our sleep. This is partly because there has been a lot of research showing that our episodic memory, or memory for specific details and events, is better after a period of sleep. For example, if you were to go out on the town and attend a show, your memory for the details and events of that show would be better the next day if you got eight hours of sleep, as opposed to staying out in the city all night. One reason for this phenomenon is that a function of sleep is consolidation (Diekelmann & Born, 2010), or the neural process by which memories are strengthened and more permanently stored. The more sleep you get, the more consolidation occurs, and the better your memories become.
There has been a lot of research regarding the effects of sleep on episodic memory, but most of this research has investigated the effects of sleep when encoding, the process of inputting information into memory, occurs before we sleep. That is, researchers have investigated how differences in sleep affect the recollection of episodic memories if the participant encodes the information, sleeps, and is then tested. It is now pretty generally accepted that sleeping between the encoding and testing of information will help you better retrieve that information from long-term memory (McBride and Cutting, 2016), which is our long-term storage of information after it has been processed.
But what are the effects of sleep on our episodic memory when the encoding occurs after sleep? Going back to the example of the show, if you were really tired while attending the show, would your episodic memory be worse than if you were well rested and wide awake? On the way home, would you have more trouble remembering the color of the characters’ costumes than if you were well rested? There is much less research regarding the effects of sleep on episodic memory when the sleep occurs prior to encoding.
The Bank Robber
In a recent study entitled “The Effects of Recent Sleep Duration, Sleep Quality, and Current Sleepiness on Eyewitness Memory” (2013), Dr. Craig Thorley of the University of Liverpool’s School of Psychology set out to investigate this very question. He investigated specifically the effects of participants’ current sleepiness, the duration of their previous night’s sleep, and the quality of that sleep on their eyewitness memory (a form of episodic memory). He did so by examining how these three sleep-related variables affected both the recollected amount (quantity) and accuracy of specific central and peripheral details of a crime scene video. In this study, central details were those that were in the center or foreground of the video and peripheral details were those that were in the background. It is important to note that Thorley did not instruct certain participants to sleep more or less than others before the study. Therefore, all of the variations in sleep duration, quality, and current sleepiness were natural.
The procedure involved the participants first rating their current sleepiness levels using the Stanford Sleepiness Scale (see below). After finishing the questionnaire, participants watched a short video of a bank robbery. The video, which was a clip from the movie “The Stick-Up,” is a classic bank robbery, executed to perfection. To watch the video, click here. After the video, participants completed the St. Mary’s Hospital Sleep Questionnaire, which assessed the duration and quality of the participants’ previous night’s sleep. Then, the participants’ eyewitness memory was tested using a 16-question memory test.
The Stanford Sleepiness Scale
Eight questions related to central details, such as “did the robber wear beige overalls?” (Thorley, 2013) and eight related to peripheral, or background details of the video. Participants answered either “yes,” “no,” or “I don’t know” to the 16 questions. By answering, “I don’t know” the participant opted out of that question. Using the results of the test, Thorley measured both the recollection quantity and accuracy of the participants’ eyewitness memory for central and peripheral details. Recollection quantity, or the proportion of information correctly remembered, was measured by dividing the number of correct answers out of 16. Recollection accuracy is a bit different. Recall that participants were allowed to answer, “I don’t know,” thus opting out of the question. Recollection accuracy is the proportion of answered questions that are correct. So, recollection accuracy was calculated by dividing the number of correct answers by the number of questions that were answered with “yes” or “no” (not opted out of). For example, if a participant answered only 10 questions with “yes” or “no” and got 8 correct, his or her accuracy would be 80%.
Contrary to what you may have thought, Thorley found that when tested only a few minutes after witnessing the crime, sleep duration, sleep quality, and current sleepiness (all prior to encoding) seem to have a fairly minor effect on overall episodic, eyewitness memory. In terms of the central, foreground details of the crime, he found that neither previous sleep duration, sleep quality, nor current sleepiness seemed to have an effect on how many central details were recalled (quantity) or how accurate those recollections were (accuracy). In terms of peripheral details, he found that none of the sleep-related variables seemed to affect how many peripheral details (quantity) participants could remember. However, Thorley did find that participants’ current sleepiness levels and the quality of their previous night’s sleep seemed to affect how accurate their recollection of peripheral details was. The lower the quality of the participants’ previous night’s sleep and the higher their current sleepiness levels were, the lower their peripheral detail recollection accuracy was.
Well, I guess that’s good right? It seems that if you’ve slept very little, very poorly, or feel very tired prior to encoding the details of a crime, it won’t have an effect on how well you can remember the central details of what happened. However, what about if you were asked to testify a week later? Would these pre-encoding sleep factors seem to affect the your recollection of crime details then? Thorley’s study tested the participants’ episodic memory of the crime scene extremely soon after they encoded the information. It would be interesting to see if the participants’ recollection quantity and accuracy of the crime details would change if the testing was delayed. 
Also, eyewitness testimonies need to be taken very cautiously. When it comes to these testimonies, the slightest of inaccuracies can cause an innocent person a lot of harm or let a criminal get off free. For example, keeping with the video that Thorley used in his study, let’s say there was an accomplice of the clown that was sneakily helping with the robbery. This would have been a peripheral detail of the crime scene and according to Thorley’s results, could have been inaccurately recollected if the witness called to testify was very sleepy during the incident. This seemingly small inaccuracy could lead to that accomplice walking the streets. The point is, although the results from Thorley’s study show that sleep quality and current sleepiness seem to only have an effect on peripheral detail recollection accuracy, it is still very important to look at whether or not an eyewitness has had poor sleep quality or is sleepy at the time of the crime. He or she could have errors in his or her recollection of the peripheral, yet still very important details.
There is still relatively little research regarding the effects of sleep of episodic memory when differences in sleep occur before the encoding of information. Therefore, more research needs to be done in this realm of memory research. However, Thorley’s findings suggest that if you do witness a crime when you are tired, or slept poorly the night before, and are asked to recall what happened soon after, you should warn the investigator that the accuracy of the peripheral details you report may not be very accurate. But your recollection of the central details should be pretty good!
To read the entire study by Dr. Craig Thorley, click here.
To read a related blog post regarding executive functioning and sleep, click here!
To read a blog post regarding sleep deprivation and false memory, click here!
To read blog more CogBlog posts on memory in general, click here!
References:
Thorley, C. (2013). The Effects of Recent Sleep Duration, Sleep Quality, and Current Sleepiness on Eyewitness Memory. Applied Cognitive Psychology, 27, 690-695.
McBride, D., & Cutting, J. (2016). Cognitive Psychology: Theory, Process, and Methodology. Sage.
Images (in the order they appear):
Image #1 was retrieved from here.
Image #2 was retrieved from here.
Image #3 was retrieved from here.
Image #4 was retrieved from here.
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