Emotional Memory Consolidation During Sleep:
November 26, 2013
Trauma, sleep and PTSD
Sleep-related difficulties are among the hallmark symptoms of posttraumatic stress disorder (PTSD). The PTSD diagnostic criteria according to DSM-5 include two sleep-related symptoms: experiencing nightmares or distressing trauma-related dreams, as part of the intrusion symptom cluster, and sleep disturbance, as part of the altered arousal and reactivity cluster. Problems such as nightmares or insomnia are indeed highly prevalent in PTSD (e.g., Spoormaker & Montgomery, 2008; Harvey, Jones & Schmidt, 2003) and a source of distress for many patients seeking treatment for PTSD (Krakow et al., 2004). In a large urban general population study, 70% of those afflicted with PTSD reported that sleep problems also affected them (Ohayon & Shapiro, 2000).
Disturbed sleep has been suggested as a risk factor for the subsequent development of PTSD. Subjective reports one month post-trauma predicted the development of PTSD 12 months later in motor vehicle accident survivors (Koren et al., 2004). Similarly, posttraumatic nightmares within a month post-trauma predicted PTSD at 6 months (Mellmann et al., 1995). Most of these studies assess sleep problems in trauma survivors weeks or months after trauma exposure. In notable exceptions, one large-scale prospective study on soldiers in the context of military deployment found that predeployment sleep duration and insomnia symptoms constitute risk factors for the development of psychopathology post-deployment, including PTSD (Gehrman et al., 2013). Similarly, van Liempt and colleagues (2013) reported that pre-deployment nightmares (although not insomnia) predicted PTSD symptoms at 6 months post-deployment.
Pre- and post-trauma sleep difficulties thus appear to play an important role in the evolution of later PTSD. There is a gap of direct evidence, however, regarding the impact of sleep immediately after trauma, i.e., hours or days after the traumatic event, on the evolution of PTSD symptoms. Sleep may play a key role role in processing and consolidating the freshly encoded and thus labile traumatic memory and this process is likely to take place within a narrow time window following trauma (Diekelmann, Wilhelm & Born, 2009). Interestingly, findings from rodent studies suggest that sleep deprivation can reduce the impact of traumatic brain injury, as rats sustained less damage following traumatic brain injury when they were kept awake for 24 hours after the injury (Martinez-Vargas et al., 2012). If these findings were replicated in human subjects in the context of experiencing trauma, they suggest that staying awake after trauma could reduce the impact of the traumatic experience. The memory consolidating properties of sleep remain influential over the passage of time, however. When new memories are learned or trauma memories are challenged or modified, for instance following emotional learning in trauma-focused therapy, these memories might also be consolidated during sleep. The next section will provide a brief, by no means comprehensive, basic neuroscience account of sleep's memory consolidating functions and the potential application to trauma and PTSD therapy.
The role of sleep in the processing of emotional memory: Offline memory consolidation during sleep
Sleep plays an important role in the modulation and integration of emotional memories (Walker, 2008, 2009, Wamsley and Stickgold, 2010). More specifically, sleep-dependent memory processing relies on an offline reactivation and consolidation during which new and initially labile memories encoded during wakefulness are transformed into more stable representations and gradually integrated in cortical networks of pre-existing long-term memories (Diekelmann and Born, 2010). Sleep following learning helps learning negative memories (Pace-Schott et al., 2009), it enhances habituation to emotional stimuli (Pace-Schott et al., 2011), or retention of previously encoded emotional pictures or texts (Nishida et al., 2009, Payne et al., 2008, Wagner et al., 2001). In addition to strengthening of the content of the emotional memory, sleep is thought to reduce the degree of emotional arousal associated with the memory (Pace-Schott et al., 2011; van der Helm et al., 2011; Walker and van der Helm, 2009). This raises the question whether sleep in the early aftermath post-trauma is either beneficial- by reducing the degree of emotional arousal of the trauma memory- or whether it could e deleterious- by further strengthening a distressing memory. There is initial evidence that memories are "tagged", so that emotional and otherwise significant memories have a processing advantage and that those pieces of an experience that are most closely tied to the affective response are preferentially processed (Bennion et al., 2013).
It is not yet clear, which sleep stages contribute to these consolidation effects. Non-rapid eye-movement (NonREM) sleep has been associated with the strengthening of hippocampus dependent declarative memories (Stickgold, 2009), whereas REM sleep has been implicated in the modulation of emotional memories and arousal (van der Helm et al., 2011b, Walker and van der Helm, 2009) and the extinction of conditioned fear (Spoormaker et al., 2011, Spoormaker et al., 2010).
Taken to the context of trauma and PTSD, sleep may play an important role for trauma memory formation and subsequent PTSD symptoms. More specifically, sleep may (i) contribute to the formation of traumatic memories in the aftermath of a traumatic event, as the initial memory traces are likely to be consolidated during sleep post-trauma, and (ii) contribute to the formation of modulated and altered memories following PTSD therapy. This will be discussed in turn, with a focus on sleep's implication for trauma-focused psychotherapy.
Translating basic neuroscience findings on sleep and memory consolidation to psychotherapy
Numerous studies posit that sleep after learning benefits memory consolidation. This knowledge is starting to be applied to the context of emotional memory and emotional learning in psychotherapy. In a recent study on phobia, we showed that sleep after exposure therapy sessions, during which patients were exosed to feared stimuli in a virtual reality setting improved therapeutic effectiveness (Kleim et al., 2013). Patients who napped after exposure therapy showed greater reductions in subjective anxiety and catastrophic cognitions at follow-up testing as compared to those who remained awake after the therapeutic intervention. One important finding of the study concerned the feasibility of a sleep intervention following exposure therapy. Exposure sessions have substantial anxious arousal properties, which might have prevented individuals from falling asleep after therapy due to sustained arousal. This was not the case, though, as we employed polysomnographic recordings and found that mean sleep onset was around 20 minutes. Individuals slept 50 minutes on average during an assigned 90-minute sleep period just after the exposure session. Napping post-therapy may be one method of enhancing exposure-based interventions in order to increase response rates. Advising patients to nap following psychotherapy may be beneficial as novel memories are formed during a session and sleep may increase their consolidation and possibly generalization (Nishida et al., 2009, Pace-Schott, 2012). These findings warrant further investigation and replication, but they suggest the merit of such a procedure for the treatment of patients afflicted with other disorders, including PTSD.
In the treatment of PTSD, exposure to the trauma memory is often used to promote emotional processing, which, in turn, is viewed by experts as the essential ingredient for treating PTSD (Foa & Kozak, 1986). Many modules of the current psychotherapies for PTSD include exposure to the trauma memory (e.g., Foa, 1992; Resick & Schnicke, 1993), possible direct exposure to the sight oft he trauma (e.g., Ehlers et al., 2005), or other elements of exposure. The above findings would suggest that sleep following exposure sessions in trauma-focused psychotherapy could further boost the effectiveness of these therapies.
Application of this basic neuroscience finding to the applied psychotherapy context is yet in its infancy, and many factors remain unexplored. For instance, it is unclear how many therapy sessions would need to be followed by sleep to reach optimal effects, or which sleep duration is most suitable. Future research will have to determine whether sleep has a generally beneficial impact on memory consolidation during psychotherapy or wether this benefit is restricted to certain types of psychotherapy sessions, such as exposure sessions. More empirical evidence is needed before recommendations for general practice and other applied settings can be given. If replicated, the new findings may offer an important non-invasive alternative to recent attempts to facilitate therapeutic memory consolidation processes and enhance psychotherapy with pharmacological interventions.
Future implications for trauma-focused psychotherapy
Sleep may enhance the effectiveness of psychotherapy by consolidation of emotional memory. This finding has recently been transfered from basic neuroscience to the context of cognitive behavior therapy and may open up new avenues for further increasing the effectiveness of trauma-focused psychotherapy. This translational approach is likely to hold much promise for the treatment of PTSD. A number of patients still do not benefit from our most successful trauma-focused therapies (Bradley et al., 2005), there is thus room for improvement. Napping after exposure to the traumatic memory or the sight of the trauma as part of trauma-focused psychotherapy, or following a session of trauma-focused virtual reality exposure therapy could further enhance the effectiveness of these psychotherapies.
About the Author
Birgit Kleim, PhD, received her doctorate in psychology from King’s College London, is a Chartered Clinical Psychologist of the British Psychological Society, and currently is a Senior Researcher at the University of Zurich where she heads the “Stress Resilience” research group funded by the Swiss National Science Foundation. Her research interests include the diagnosis, screening and prediction of chronic posttrauma psychopathology, understanding resilience to chronic stress and traumatic events, and psychotherapy research.
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