The Effect Of Sleep On Intrusive Memories in Daily Life: A Systematic Review And Meta-analysis Of Trauma Film Experiments Part 1
Oct 19, 2023
Abstract
Study Objectives:
To synthesize the literature on the effect of sleep versus wake on the frequency and distress of intrusive memories in everyday life after watching film clips with distressing content as a proxy for traumatic experiences.
An intrusive memory is a form of memory that is so intensely imprinted on our brain that it is often difficult for us to forget or forget about it. There may be some people who think about some bad experiences that happened to them, which don't make people feel good, but they can help us grow. However, for others, intrusive memories may cause them more pain and distress, especially if they are caused by a traumatic event.
However, some research suggests that intrusive memories may have a positive impact on our memory. In the study, researchers monitored the effects of intrusive versus non-intrusive memories in the brain and found that intrusive memories appear to improve our memory.
The reason for this improved memory may be related to the characteristics of the intrusive memory itself. They can be very vivid and deeply imprinted in our minds, which can help us recall important information more easily. Additionally, because intrusive memories are often triggered by memorable situations we experience that may be related to the information we learned, they may act like a "memory frame." Additionally, since intrusive memories, are already linked to our emotional and physiological responses, they better stimulate different areas of our brains and help us better focus on the memory process.
Overall, while intrusive memories may cause some discomfort and pain, on a positive note, they may also affect our memory, making it easier for us to recall important information and become a better person. It can be seen that we need to improve our memory. Cistanche deserticola can significantly improve memory because Cistanche deserticola is a traditional Chinese medicinal material with many unique effects, one of which is to improve memory. The efficacy of minced meat comes from the various active ingredients it contains, including acid, polysaccharides, flavonoids, etc. These ingredients can promote brain health in various ways.
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Methods:
We conducted a systematic review by searching PubMed and PsychInfo. The last search was conducted on January 31, 2022. We included experimental studies comparing sleep and wake groups on intrusions using ecological diary methods, whereas studies lacking a wake control condition or relying solely on intrusion-triggering tasks or retrospective questionnaires were excluded. Meta-analyses were performed to evaluate the results. Risks of biases were assessed following the Cochrane guidelines.
Results:
Across 7 effect sizes from 6 independent studies, sleep (n = 192), as compared to wake (n = 175), significantly reduced the number of intrusive memories (Hedges’ g = −0.26, p = .04, 95% CI [−0.50, −0.01]), but not the distress associated with them (Hedges’ g = −0.14, p = .25, 95% CI [−0.38, 0.10]).
Conclusions:
Although the results suggest that sleep reduces the number of intrusions, there is a strong need for high-powered pre-registered studies to confirm this effect. Risks of biases in the reviewed work concern the selection of the reported results, measurement of the outcome, and failure to adhere to the intervention. Limitations of the current meta-analysis include the small number of studies, which comprised only English-language articles, and the fact that it was not pre-registered.
Keywords:
Sleep; intrusions; memory; emotion; trauma film paradigm; PTSD; involuntary memories.
Statement of Significance
The current meta-analysis suggests that sleep, compared to waking, following negative emotional experiences reduces the number of intrusive memories of these experiences. Should this finding be robustly replicated in high-powered studies, this would indicate that sleep should be actively promoted in the immediate aftermath of negative emotional experiences, as this could serve a protective function that reduces their negative consequences. After extensive well-powered replication of this finding in samples with healthy participants using lab versions of traumatic experiences, the next step would be to start examining sleep-promoting interventions after real traumatic experiences. Sleep-promoting interventions are especially promising given that they are cost-effective and easy to deliver, meaning they would be applicable on a mass scale.
Introduction
Exposure to traumatic events is often followed by involuntary, recurrent, and distressing memories of them (i.e. intrusions [1]). Both intrusions and sleep disturbances are considered hallmark symptoms of post-traumatic stress disorder (PTSD) [2].
Sleep has been found to play a beneficial role in memory consolidation [3–5]. For instance, recent meta-analytic work has revealed that sleep, as compared to wake, leads to enhanced memory performance for material encoded before sleep manipulation [3, 4]. Sleep has also been suggested to moderate emotional reactivity to stressful events, although the direction of such changes has varied between studies [6].
Although research on the effect of sleep on emotional memories has typically focused on the intentional retrieval of explicit memories [7–9], or emotional reactivity to reminders of the affective stimuli [6], recent research has begun examining the effect of sleep on involuntary intrusions of negative experiences [10]. This meta-analysis aimed to synthesize the experimental literature on the effect of sleep on intrusive memories.
Traumatic events can have long-lasting effects on people’s health and daily functioning (e.g. [11]). Not enough is yet known about the effects of sleep deprivation following distressing experiences to begin experimentally manipulating sleep after actual traumatic events or other negative experiences. Thus, work on this topic so far has been conducted using milder proxies of traumatic events, such as watching film clips depicting car crashes, bodily harm, or physical assaults, an experimental paradigm often referred to as the trauma film paradigm [12]. In this paradigm, participants first watch a trauma film and are then asked to keep a diary for a certain period (typically 7 days).
In this diary, they are asked to record all intrusive memories of the film as soon as they experience them and to rate the degree of distress associated with each intrusion. To examine the effect of sleep on intrusions, sleep is manipulated after viewing the trauma film. This could for instance be done by having one group sleep as normal the following night and another group undergo a night of total sleep deprivation, or by having one group take a daytime nap and another group be awake for an equivalent amount of time.
The first study to examine the effect of sleep on intrusions using the trauma film paradigm found fewer intrusions during the week following a night of sleep deprivation as compared to after a night of regular sleep [10]. The authors interpreted this finding to suggest that sleep deprivation disrupts the consolidation of memories that normally occurs during sleep, leading to a reduced number of involuntary memories as well. Shortly after that paper was published, another study found an effect in the opposite direction, with fewer intrusions following sleep as compared to waking [13].
The authors interpreted this as sleep-promoting the consolidation and integration of the memory, arguing that without sleep, the experience is “predominantly laid down in memory in a disorganized and fragmented fashion that is not well integrated into its context in time, place, subsequent and previous information, and other autobiographical memories” [13; p. 2192]. This lack of integration would then make the memory more intrusive and distressing. Since then, two more studies have also found sleep to reduce the number of intrusions [14, 15], whereas two other studies have found no group differences [16, 17] (for narrative reviews, see [6, 18, 19]). Based on the mixed findings across these studies, a meta-analysis could clarify the current status of this literature and inform future research on this topic by examining the emergent pattern of the association between sleep and intrusions with all studies combined. The previous literature has yielded two conflicting hypotheses regarding intrusion frequency.
The first hypothesis states that sleep increases the number of intrusions by consolidating the memory in a manner that increases its accessibility and facilitates both voluntary and involuntary retrieval. The second hypothesis states that sleep decreases the number of intrusions by consolidating the memory in a manner that makes it less fragmented, which makes it less likely to be intrusive. By performing a meta-analysis, we sought to examine which of these hypotheses is best supported by the empirical work on this topic so far.
Beyond the number of intrusions, it is also important to consider the distress associated with them, given that all intrusive memories are not necessarily distressing [20]. The distress associated with intrusive memories is also considered a diagnosis criterion for PTSD [2] and has been suggested to be more predictive of PTSD severity than the frequency of intrusions [20, 21]. In the context of the trauma film paradigm, Kleim et al [13] found sleep to decrease the average distress of the intrusions, whereas none of the other studies has revealed any group differences [10, 14–17]. In addition to intrusion frequency, we included distress in the meta-analysis to examine if any clear pattern would emerge when aggregating all studies together. We did not have a directed hypothesis for the effect of sleep on intrusion distress.
We included studies that compared sleep to wake and measured spontaneous, involuntary memories in daily life as the outcome. The objective of this systematic review and meta-analysis was to examine if sleep, compared to wake, decreases the number of intrusions spontaneously occurring in people’s everyday lives, and the average distress associated with them, in response to films with trauma-related content.
Methods
Literature search and inclusion criteria
The literature review for this meta-analysis followed the PRISMA guidelines (see the PRISMA 2020 Checklist in the supplemental material). The first author has previously published a narrative review on this topic [6], concluding that there were contrasting results in the literature. The present study sought to follow up on this by performing a meta-analysis and risk-of-bias assessment. The present meta-analysis was not pre-registered.
We performed a systematic search of the databases PsycInfo and PubMed, using the search terms Sleep AND Memory AND Intrusi*. The last search for both databases was conducted on January 31, 2022. To be included in the meta-analysis, a study needed to compare a sleep and a wake group on a measure of spontaneous intrusions occurring in everyday life (i.e. outside the laboratory) recorded by the participants in some form of diary. In other words, studies examining if sleep increased intrusions during any kind of intrusion-triggering task [22, 23] were not included. Neither did we include effects related to general intrusion symptomology as measured by retrospective questionnaires [10, 15–17]. The rationale for excluding questionnaire results was that the questionnaires were administered on different days during the week, making it difficult to compare the studies. We only included papers published in peer-reviewed journals.
The literature search generated a total of 130 unique entries. The first author reviewed the abstract of each entry. About 112 papers were excluded during this process because they were not empirical papers or because they had not examined the effect of sleep on intrusions, leaving 18 papers for which the first author reviewed the full manuscript. Of these remaining papers, three were excluded because they had not examined the effect of sleep on intrusions, six were excluded because they did not include a wake control group, and three were excluded because they did not measure spontaneous intrusions in daily life but only included lab-based intrusion-triggering tasks, leaving 6 papers containing a total of 7 effect sizes for the final meta-analyses. For an overview of this process, see the flowchart in Figure 1.
The first author collected the relevant data (n, M, and SD) for each group in each study for both intrusion frequency and distress. In addition, the first author extracted the type of sleep manipulation (e.g. naps during the day or regular sleep at night), sex and age of participants, and the number of participants reporting zero intrusions during the experimental week.
If this information was not available, the first author contacted the corresponding author of the paper in question. We were interested in aggregated group differences across all study days, and not in day-by-day interactions (some studies have reported data for intrusion frequency on a day-by-day basis whereas others have not). Table 1 summarizes the studies included in the analyses.
We included all studies that have contrasted a sleep and a wake group as long as all other factors were held equal between groups. As shown in Table 1, three studies compared nighttime sleep with nighttime sleep deprivation, one compared nighttime sleep to daytime wake or nighttime sleep deprivation (the latter two collapsed into one group for the analysis), and two compared nap designs (three datasets in total). The rationale for including all these categories was to increase power by raising the sample size and that studies with short sleep durations should be able to reveal differences between sleep and wake. We modeled the effects as random to allow differences across studies.
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