That conclusion is reported by Dr. Ed Geiselman, a UCLA psychology professor and a faculty member for the Force Science Analysis certification course, after assessing the findings from a series of experiments about memory.
"It's generally presumed that memory is best mined when it is freshest," Geiselman told Force Science News recently, "and before it can be 'contaminated' by input from other sources, rationalization, mood change, change of setting, and the normal deterioration over time."
But Geiselman decided to re-examine this premise after Dr. Bill Lewinski, executive director of the Force Science Institute, mentioned to him that when a department insists on formally interviewing an involved officer before releasing him or her after a shooting, the officer sometimes has been awake for 24 to 36 hours or more when questioned.
Geiselman dug back into data he'd collected several years ago during 3 research projects involving some 600 eyewitnesses. In these experiments, civilians were unexpectedly exposed to what they thought was an authentic, sudden assault involving live actors (the incident was actually staged) or they variously viewed videotape of a real or simulated robbery or purse snatching.
Later the subjects' abilities to verbally describe participants, identify them from photo lineups, and to recount as much as they could of the action from start to finish ("free recall") were tested.
Just before the tests, the subjects completed a detailed questionnaire. One of its "many items" was: "How well rested are you right now?" They were asked to rate themselves on a scale of 1 to 5, from "not at all rested" to "very well rested."
It was these answers, which he previously had not considered in isolation and had not reported, that Geiselman now focused on as he re-analyzed the data.
"I found a very strong correlation between rest and memory," he says. "In each test, the people who reported being well rested did significantly better than those who weren't. Their verbal descriptions were more detailed and more accurate, their lineup IDs were more reliable, and their narratives of the action they had witnessed were far more thorough and correct."
And these were results just from relatively low-stressed, passive observers. "You would expect police officers who've been physically and emotionally involved in a high-stress, life-threatening encounter to experience an even more pronounced effect on their memory from fatigue or rest," Geiselman says.
"Clearly the findings are consistent with the idea that allowing an officer to rest before being interviewed is an important consideration. Rest likely plays a causal role in how well you are able to remember."
Other researchers have found an additional negative connection between lack of rest and damaged memory. "Recent research suggests that sleep deprivation may contribute to the generation of false memories," Geiselman notes in an article on his findings that appears in the current edition (vol. 28, issue 2, 2010) of the American Journal of Forensic Psychology.
"We're not talking about deliberate lies," he says, "but about involuntary distortions caused by biochemical reactions in the brain to sleep loss that cause you to remember things differently than what really happened. In short--more sleep deprivation, more errors."
Considering all factors, "Waiting a day or 2 for an officer to be better rested before being interviewed extensively about a shooting should not be problematic," Geiselman suggests. "Any lost memory during that time period should be recoverable because the officer will be in better condition emotionally, physiologically, and cognitively to participate."
It is believed that deep sleep "plays an important role in the consolidation of memories," thereby making recall "more complete," Geiselman writes. "The brain does a lot of work while you are sleeping."
Lewinski, among others, has recommended that officers be allowed 2 sleep cycles--perhaps even longer in some cases--before having to write a formal statement or submit to an interview about any life-threatening event.
"If an officer is tired, his ability to extract memory is impaired, while the quality of memory is enhanced by sleep," he explains. "Rest not only helps an officer respond better, providing more information more accurately, but also helps him be more in tune with the interview. He can better avoid distractions and better understand what is being asked and what his answers should be to be relevant and comprehensive.
Rest also, of course, allows for some emotional decompression. The stress of the incident has some time to fade before the officer has to relive that stress in the interview."
The challenge, Lewinski and Geiselman agree, may be in getting sufficient rest even when given waiting time to do so. OIS researcher Dr. David Klinger, himself an ex-cop, has pointed out that 46% of officers involved in shootings experience difficulty sleeping within the first 24 hours afterwards. For about one-third, sleep problems persist even after 1 week.
Even if some fatigue remains, Geiselman says, recall will be maximized if officers are questioned by investigators employing cognitive interviewing techniques. This "highly recommended" approach, Geiselman explains, incorporates methods "for reconstructing and reinstating the sensory and emotional context that existed at the time" of the shooting and for "enhancing memory retrieval following some forgetting."
Cognitive interviewing also has been "found to circumvent certain post-event, contaminating influences," thereby helping to "counter any negative effects on memory caused by delay," he says.
In the future, Geiselman hopes to launch experiments that concentrate specifically on rest and memory. "We need to systematically manipulate the length and nature of rest after a critical incident and see how recall is affected. Then we should be able to pinpoint more precisely what level of rest seems most productive.
"Meanwhile, the expression 'let me sleep on it' appears to have validity as it applies to memory recall performance."