Narcolepsy - A hypersomnia characterized by profound sleep-onset paralysis and vivid hallucinations that occur as a person is falling asleep (hypnagogic hallucinations). Narcolepsy with cataplexy also involves sudden loss of muscle control (cataplexy).
Night Eating Syndrome (NES) - A condition in which an individual feels hungry, even if he or she has just eaten, and is unable to go to sleep without eating again. NES is not a parasomnia because it occurs when the individual is completely awake. It is different from Sleep-Related Eating Disorder (SRED), which is a parasomnia.
Nightmare - Disturbing mental experiences that generally occur during Rapid Eye Movement (REM) sleep in the later portion of the night, and often result in the person waking up. Occasional nightmares are normal; a nightmare disorder is diagnosed only if nightmares become more frequent and significantly affect daytime functioning and mood.
Non-24-Hour Sleep Wake Disorder - A Circadian Rhythm Disorder characterized by an abnormal sleep-wake rhythm caused by a circadian clock that is not in phase with, and cannot be entrained to, the 24-hour light-dark cycle. It is most frequently experienced by individuals who are blind.
Non-Rapid Eye Movement (NREM) Sleep - One of the three basic states of consciousness, characterized by a reduction in physiological activity. As sleep deepens, a personís brain waves slow down and gain amplitude, both breathing and the heart rate slow down, and the individualís blood pressure drops.
Non-Rapid Eye Movement (NREM) Sleep Arousal Disorders - A parasomnia that occurs when the brain is partly in Non-REM sleep and partly awake enough to perform complex activities without any conscious awareness of them. NREM disorders include confusional arousals, sleep-walking, sleep terrors, sleep sex (sexsomnia), and Sleep Related Eating Disorder (SRED).
Noninvasive Positive Pressure Ventilation (NIPPV) - Ventilation applied through a mask rather than through an endotracheal tube. This type of ventilation has a backup mode that delivers automatic pressure changes for inhalation and exhalation when patients do not trigger such pressure changes themselves.