RBD associated with neurological disorders is frequently related to abnormal accumulation of alpha-synuclein, and more than 80% of patients with idiopathic RBD might develop Lewy body disease (LBD). Patients with narcolepsy are also more likely to develop RBD. The diagnosis is based on clinical history, including partners account and needs to be confirmed by polysomnography (PSG), mainly for its accuracy in differentiating RBD from other sleep disorders, since there is a loss of REM atonia with excessive muscle tone. However, screening questionnaires, such as RBDSQ, are also very useful for diagnosing RBD. Recurrent isolated sleep paralysis
Recurrent isolated sleep paralysis is an inability to perform voluntary movements at sleep onset, or upon waking from sleep. Although the affected individual is conscious and recall is present, the person is not able to speak or move. However, respiration remains unimpaired. The episodes last seconds to minutes and diminish spontaneously. The lifetime prevalence is 7%. Sleep paralysis is associated with sleep-related hallucinations. Predisposing factors for the development of recurrent isolated sleep paralysis are sleep deprivation, an irregular sleep-wake cycle, e.g. caused by shift work, or stress. A possible cause could be the prolongation of REM sleep muscle atonia upon awakening. Nightmare disorder
Nightmares are like dreams primarily associated with REM sleep. Nightmare disorder is defined as recurrent nightmares associated with awakening dysphoria that impairs sleep or daytime functioning. It is rare in children, however persists until adulthood. About 2/3 of the adult population report experiencing nightmares at least once in their life.
Parasomnias  are a category of sleep disorders that involve abnormal movements, behaviors, emotions, perceptions, and dreams that occur while falling asleep, sleeping, between sleep stages, or during arousal from sleep. Parasomnias are dissociated sleep states which are partial arousals during the transitions between wakefulness, NREM sleep, and REM sleep, and their combinations. Classification
The newest version of the International Classification of Sleep Disorders (ICSD, 3rd. Ed.) uses State Dissociation as the paradigm for parasomnias. Unlike before, where wakefulness, non-rapid eye movement (NREM) sleep, and rapid eye movement (REM) sleep were considered exclusive states, research has shown that combinations of these states are possible and thus, may result in unusual unstable states that could eventually manifest as parasomnias or as altered levels of awareness.Although the previous definition is technically correct, it contains flaws. The consideration of the State Dissociation paradigm facilitates the understanding of the sleep disorder and provides a classification of 10 core categories. Non-rapid eye movement (NREM)-related parasomnias
NREM parasomnias are arousal disorders that occur during stage 3 (or 4 by the R&K standardization) of NREM sleep—also known as slow wave sleep (SWS). They are caused by a physiological activation in which the patients brain exits from SWS and is caught in between a sleeping and waking state. In particular, these disorders involve activation of the autonomic nervous system, motor system, or cognitive processes during sleep or sleep-wake transitions.Some NREM parasomnias (sleep-walking, night-terrors, and confusional arousal) are common during childhood but decrease in frequency with increasing age.