Melbourne School of Psychological Sciences - Theses
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ItemSleep and PTSD: bi-directional relationship and underlying mechanism( 2023-04)Post-traumatic stress disorder (PTSD) is a debilitating and enduring disorder that a small but significant number of people develop following exposure to a traumatic event. A common feature of PTSD is disrupted sleep including insomnia and nightmares. Ongoing difficulties in sleeping prevent the sleep-dependent adaptive processing of traumatic memories. Further, sleep disruptions prevent recovery, and perpetuate the disorder when established. This thesis aimed to investigate the bi-directional association between changes in sleep and PTSD symptoms as well as between sleep and underlying fear memory processes. In part I, I examined the immediate effect of changes in sleep on fluctuations of PTSD symptoms and vice versa. Here I used both subjective and, for the first time, objective measures of sleep (study 1). This study found differences in the association between sleep and daytime PTSD symptoms depending on the sleep measurement method. Additionally, preliminary evidence suggests sex-specificity in the association between night-time sleep and daytime PTSD symptoms. Part II focused on the role of sleep in fear conditioning and extinction learning – the experimental model of PTSD development and treatment. Extinction learning and extinction recall (i.e., the ability to learn and remember that previously dangerous stimuli are not threatening anymore) are thought to be impaired in PTSD and impacted by sleep. Particularly rapid eye movement (REM) sleep has been suggested as the sleep stage most important for processing emotional memories. First, a systematic review summarized the available literature assessing the effect of REM and other sleep stages using a meta-analytic approach (study 2). The overwhelming majority of research highlighted the importance of REM sleep, but the meta-analysis did not find the expected REM sleep effect on extinction recall. Following this and the finding from part I, the third and last study investigated the effect of subjectively reported sleep on fear conditioning and extinction learning (study 3). Again, no effect of subjective sleep indices on extinction recall was found. However, both studies found that there were differences between sexes and outcomes were dependent on the diagnostic status (study 2) or rather PTSD severity (study 3). Together, this thesis filled important gaps in the literature and highlighted that sleep is important in the expression of PTSD symptoms in day-to-day life as well as in the mechanism underling PTSD such as fear conditioning and extinction learning. However, the effect of sleep may vary depending on key associated factors. Specifically, the relationship may differ depending on the sleep measurement method (objective and subjective), the samples studied (clinical and control populations) and sex (men and women).
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ItemSleep, mood, and cognitive vulnerability in adolescents: a naturalistic study over restricted and extended sleep opportunities( 2013)Introduction: It is well established that for adolescents, school days are associated with sleep restriction, and that insufficient sleep has been linked to mood disturbances. This longitudinal study assessed sleep, mood, and life stress over the school term and vacation periods with restricted and extended sleep opportunities. The relationships between objective and subjective sleep, as well as between sleep and mood were examined. A cognitive model was proposed and tested to assess whether sleep-specific (i.e., dysfunctional beliefs and attitudes about sleep) and global (i.e., dysfunctional attitudes) cognitive vulnerabilities played a role in these relationships. Methods: One-hundred and forty-six adolescents (47.3% male) aged 16.2+/-1.0 years (M+/-SD) from the general community wore an actigraph continuously for four weeks: the last week of a school term (Time-E), the following two-week vacation (Time-V), and the first week of the next term (Time-S). Social demographic information, chronotype, and cognitive vulnerabilities were assessed at Time-E. Subjective sleep, symptoms of depression, anxiety, and life stress were repeatedly measured at Time-E, Time-V, Time-S, and the middle of the subsequent school term. Regression analyses were used to explore the relationship between sleep and mood, and structural equation modelling was used to examine changes of variables over time, as well as the moderating roles of cognitive vulnerabilities. Results: Compared with school days, sleep during the vacation was characterized by later timing, longer duration, lower quality and greater variability. Daily changes in actigraphy- measured sleep over the vacation period showed linear delays in sleep timing throughout the vacation, while changes in time-in-bed were non-significant. The first vacation week was characterized by a linear decrease in total sleep time and sleep quality, and these changes stabilized during the second vacation week. Compared to vacations, school terms were associated with higher symptoms of depression, anxiety, and life stress. Poorer sleep quality, particularly poorer subjective perception of sleep quality, was significantly associated with higher symptoms of depression and anxiety. Sleep- specific cognitive vulnerability moderated the relationship between objective and subjective sleep onset latency during extended but not restricted sleep opportunity. After controlling for life stress, global cognitive vulnerability played different moderating roles in the relationship between subjective sleep and mood over school term and vacation periods. Higher global cognitive vulnerability was associated with a stronger relationship between subjective sleep and symptoms of anxiety (but not depression) during the school term, as well as with a stronger relationship between subjective sleep and symptoms of depression (but not anxiety) during the vacation period. Conclusion: Sleep, mood, and life stress changed markedly over the school term and vacation periods. Changes in sleep over the vacation suggested that the recovery from school- related sleep restriction was completed within two weeks’ extended sleep opportunity, and the average sleep duration over this period suggested that sleep requirements in adolescence may be less than conventionally described in the media and in the scientific literature. Cognitive vulnerabilities played important roles in the relationship between sleep and mood. Adolescents with higher cognitive vulnerability might be more emotionally vulnerable towards school-related sleep restriction. These findings have important implications for future studies, as well as practical implications for policies and interventions designed to improve adolescents’ wellbeing.