Sleepy Brains, Dreamy Minds: A Neurophysiological Exploration of Conscious Experiences in Sleep Disorders

Dreams are a fundamental component of human sleep. Defined broadly, they encompass a wide range of subjective experiences, including fleeting images, abstract thoughts, and vivid sensory experiences with a narrative structure. For most individuals, such experiences are transient, forgotten upon awakening, and without lasting consequences. For others, however, mental activity during sleep can become a source of distress or impairment. Nightmares, ruminative thoughts that render sleep non-restorative, frightening hallucinations, or relentless dreaming illustrate how sleep-related experiences may disrupt sleep quality. Despite their clinical relevance, mental activity during sleep remains markedly underassessed in practice. Few sleep professionals inquire systematically about dreams or sleep-related experiences, and standard polysomnographic evaluations do not incorporate structured assessments of subjective experience. Consequently, both the nature of these complaints and their underlying neural mechanisms remain poorly understood. This thesis builds on a paradigm shift in sleep research that conceptualizes sleep as a locally modulated and spatially heterogeneous brain state, and on previous work identifying regional electrophysiological correlates of dream content with high-density electroencephalography (hd-EEG). It proposes that controlled assessments of sleep-related mental activity, combined with appropriate recording techniques, can reveal objective brain correlates of subjective experience that remain undetected by standard sleep recordings. The work focuses on two conditions in which altered mental activity during sleep is central yet poorly understood: non-rapid eye movement (NREM) parasomnias (sleepwalking and related disorders), and the feeling of being awake during sleep in patients with insomnia. These phenomena were investigated using serial awakenings combined with hd-EEG. In sleepwalking, phenomenological assessment revealed that the level of consciousness, degree of sensory disconnection, and extent of amnesia varied widely across episodes and individuals, forming a continuum rather than a categorical state. hd-EEG analyses identified neural correlates of this variability: parieto-occipital activation was associated with dream-like experiences, while medial temporal activation predicted memory for episode content. These findings indicate that sleepwalking reflects an incomplete and spatially heterogeneous arousal from sleep, in which regional activation patterns determine both experience and recall. Serial probing of sleep state perception showed that the feeling of being awake, in both good sleepers and patients with insomnia, was most commonly reported during early NREM sleep. Patients with insomnia additionally felt less asleep during REM sleep compared to controls, and more often described abstract or ruminative thinking rather than vivid dreaming. hd-EEG analyses revealed that the feeling of being awake was correlated with increased high-frequency activity in anterior cortical regions in good sleepers, and this correlation extended to posterior regions in patients with insomnia. Together, these findings suggest that feeling awake during sleep is supported by a physiological process that predominates early in the night and is normally attenuated during REM sleep, but is more frequently activated and abnormally persists into REM sleep in individuals with insomnia. The work presented in this thesis indicates that conscious experience during sleep is associated with regionally specific cortical activation rather than global state changes. The findings demonstrate that systematic phenomenological assessment of mental activity during sleep, combined with hd-EEG, provides critical insights into the neural mechanisms underlying sleep quality and abnormal mental activity in sleep disorders. More broadly, this work highlights the limitations of standard sleep metrics in accounting for complaints related to mental activity during sleep, underscores the importance of integrating subjective experience into sleep research and clinical practice, and contributes to a deeper understanding of the neural basis of sleep consciousness.