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Rapid eye movement (REM) sleep is characterized by the alternation of two markedly different microstates, phasic and tonic REM. These periods differ in awakening and arousal thresholds, sensory processing, and spontaneous cortical oscillations. Previous studies indicate that although in phasic REM, cortical activity is independent of the external environment, attentional functions and sensory processing are partially maintained during tonic periods. Large-scale synchronization of oscillatory activity, especially in the α- and β-frequency ranges, can accurately distinguish different states of vigilance and cognitive processes of enhanced alertness and attention. Therefore, we examined long-range inter- and intrahemispheric as well as short-range electroencephalographic synchronization during phasic and tonic REM periods quantified by the weighted phase lag index. Based on the nocturnal polysomnographic data of 19 healthy adult participants, we showed that long-range inter- and intrahemispheric α and β synchrony was enhanced in tonic REM states in contrast to phasic ones, and resembled α and β synchronization of resting wakefulness. On the other hand, short-range synchronization within the γ-frequency range was higher in phasic compared with tonic periods. Increased short-range synchrony might reflect local and inwardly driven sensorimotor activity during phasic REM periods, whereas enhanced long-range synchrony might index frontoparietal activity that reinstates environmental alertness after phasic REM periods.

Abstract Study Objectives Frequent nightmares have a high prevalence and constitute a risk factor for psychiatric conditions, but their pathophysiology is poorly understood. Our aim was to examine sleep architecture and electroencephalographic markers—with a specific focus on state transitions—related to sleep regulation and hyperarousal in participants with frequent nightmares (NM participants) versus healthy controls. Methods Healthy controls and NM participants spent two consecutive nights in the sleep laboratory. Second night spectral power during NREM to REM sleep (pre-REM) and REM to NREM (post-REM) transitions as well as during NREM and REM periods were evaluated for 22 NM participants compared to 22 healthy controls with a similar distribution of age, gender, and dream recall frequency. Results We found significant differences between the groups in the pre-REM to post-REM changes in low- and high-frequency domains. NM participants experienced a lower amount of slow-wave sleep and showed increased beta and gamma power during NREM and pre-REM periods. No difference was present during REM and post-REM phases. Furthermore, while increased pre-REM high-frequency power seems to be mainly driven by post-traumatic stress disorder (PTSD) symptom intensity, decreased low-frequency activity occurred regardless of PTSD symptom severity. Conclusion Our findings indicate that NM participants had increased high-frequency spectral power during NREM and pre-REM periods, as well as relatively reduced slow frequency and increased fast frequency spectral power across pre-and post-REM periods. This combination of reduced sleep-protective activity and increased hyperarousal suggests an imbalance between sleep regulatory and wake-promoting systems in NM participants.

Nightmare disorder is characterized by dysfunctional emotion regulation and poor subjective sleep quality reflected in pathophysiological features such as abnormal arousal processes and sympathetic influences. Dysfunctional parasympathetic regulation, especially before and during rapid eye movement (REM) phases, is assumed to alter heart rate (HR) and its variability (HRV) of frequent nightmare recallers (NM). We hypothesized that cardiac variability is attenuated in NMs as opposed to healthy controls (CTL) during sleep, pre-sleep wakefulness and under an emotion-evoking picture-rating task. Based on the polysomnographic recordings of 24 NM and 30 CTL participants, we examined HRV during pre-REM, REM, post-REM and slow wave sleep, separately. Additionally, electrocardiographic recordings of resting state before sleep onset and under an emotionally challenging picture-rating task were also analyzed. Applying repeated measures analysis of variance (rmANOVA), a significant difference was found in the HR of NMs and CTLs during nocturnal segments but not during resting wakefulness, suggesting autonomic dysregulation, specifically during sleep in NMs. As opposed to the HR, the HRV values were not significantly different in the rmANOVA in the two groups, implying that the extent of parasympathetic dysregulation on a trait level might depend on the severeness of dysphoric dreaming. Nonetheless, in the group comparisons, the NM group showed increased HR and reduced HRV during the emotion-evoking picture-rating task, which aimed to model the nightmare experience in the daytime, indicating disrupted emotion regulation in NMs under acute distress. In conclusion, trait-like autonomic changes during sleep and state-like autonomic responses to emotion-evoking pictures indicate parasympathetic dysregulation in NMs.

Abstract Study Objectives: We aimed to investigate the effect of directed forgetting instruction on memory retention after a 2-hour delay involving a daytime nap or an equivalent amount of time spent awake. We examined the associations between sleep-specific oscillations and the retention of relevant and irrelevant study materials. Methods: We applied a list-method directed forgetting paradigm manipulating the perceived relevance of previously encoded lists of words. Participants were randomly assigned to either a nap or an awake group, and to a remember or a forget subgroup. The remember and the forget subgroups were both instructed to study two consecutive lists of words, although, the forget subgroup was manipulated to forget the first list and memorize only the second one. Participants were 112 healthy individuals (44 men; Mage = 21.4 years, SD = 2.4). Results: A significant directed forgetting effect emerged after a 2-hour delay both in the awake and sleep conditions; however, the effect was more pronounced within the sleep group. The benefit of directed forgetting, that is, relatively enhanced recall of relevant words in the forget group, was evidenced only in those participants that reached rapid eye movement (REM) phase. Non-rapid eye movement (NREM) sigma power was correlated with memory performance for the relevant (second) list, and sleep spindle amplitude was associated with the retention of both lists. These associations, however, were detected only within the forget subgroup. REM duration correlated with recall performance for the relevant (second) list within the forget subgroup, and with recall performance for the first list within the remember subgroup. Conclusions: A directed forgetting effect persists after a 2-hour delay spent awake or asleep. Spindle-related activity and subsequent REM sleep might selectively facilitate the processing of memories that are considered to be relevant for the future.

Sleep can be defined behaviorally as a reversible state of perceptual disengagement from the environment during which we are unresponsive to external stimuli (to a given extent) and usually entails a species specific posture (lying down in humans) combined with closed eyes and limited amount of movements (Pelayo, 2017). There has been multiple theories throughout the history of sleep science trying to unfold what could happen during this unconscious ‘black box’ which we spend a third of our lives with. Until the 1920s the idea of a ‘switched off brain’ during sleep was widely accepted. This entirely passive view of sleep has been once and for all disproved by the first recording of human electroencephalography (EEG) (Berger, 1929) measuring on-line brain activity both during wake and during sleep. Nevertheless, the activity patterns between these two complementary states are highly divergent. These characteristic activity patterns are the result of multiple complex and dynamic mutual inhibitory interactions of the so called wake promoting and sleep promoting systems in the brain. Even though it has been almost a hundred years, EEG is still the most reliable equipment to investigate information processing during uninterrupted sleep. Although the apparatus is the same the methods used in sleep research came a long way thanks to the technical developments ever since. The invention of computer and the digitalization of the analog EEG signal gave us numerous new tools to ‘dig deeper’ into this multidimensional signal acquired by the EEG and by adapting complex signal processing and statistical methods from neighboring scientific fields – such as mathematics and physics – we are starting to look further than ‘the eye can see’, over the horizon of the macrostructural level into the depth of microstructural and spectral sleep EEG analysis.In the first part of this review I aim to summarize the biggest milestones throughout the evolution of sleep research focusing mainly on the specific aspects of the neurobiology of sleep regulation related to the processes and states investigated in the included studies (such as arousal regulation or REM and NREM regulation). I will primarily concentrate on the dynamic interaction between the sleep-promoting and wake-promoting systems since the studies presented in this thesis mostly draw conclusions in relation to the mutual inhibition processes between these systems.Based on the evidence from neurobiology of sleep mainly provided by animal studies, in the second part I will highlight the most important reasons advocating for EEG as the best method of investigating sleep and these dynamic changes between the sleep- and wake promoting system in humans. Within this section I will present the two most frequently used methods of quantitative EEG analysis (spectral analysis and phase synchronization) and provide examples throughout the five studies attached, of various ways to use these methods to investigate different aspects of information processing (e.g. memory consolidation) and arousability during sleep.1. Sleep as a passive, silent stateEarly theories assumed that sleep is an entirely passive process – a time characterized by a completely ‘switched off’ brain – similar to other quiescence states such as coma or anesthesia. The main theory (Piéron, 1913) until the 1920s hypothesized that so called ‘hypnotoxins’ are responsible for falling and staying asleep. These toxins were thought to accumulate during a day spent awake causing higher and higher fatigue levels until at some point ‘switching the brain off’ and resulting in sleep. During sleep they were thought to be eliminated resulting in wake so the whole process could start from the beginning. Even though, this theory is more than a 100 years old the basis of it is really close to what we now believe to be process S (sleep-dependent process) in the two process model of sleep regulation (Borbély, 1982). During this time in regards of sleep research all the attention was directed towards dreams and the importance of dream analysis (e.g. Sigmund Freud and his famous work ‘The interpretations of dreams’(Freud, 1900)).