The exact cause of narcolepsy is not known. An abnormality in the chemistry regulating sleep and wakefulness in the brain is suspected, but not proven. Both genetic and environmental factors are believed to play a role in the development of this disorder.  All patients experience excessive daytime sleepiness (sleep attacks and persistent daytime drowsiness). Sleep attacks are short periods of sleep that occur many times a day, regardless of the amount or quality of sleep the night before. They are often described as irresistible and may occur with or without warning when a person is driving, working, eating, talking, or engaging in any other activity. Most patients also experience persistent daytime drowsiness.   Excessive daytime sleepiness is usually the first symptom of narcolepsy (link mirrored in case it isn't available - click here) and often the most difficult symptom to control. The severity of excessive daytime sleepiness varies; some patients may have many sleep attacks each day and others o nly one or two sleep attacks per day.    Cataplexy refers to sudden, brief episodes of muscle weakness or paralysis triggered by strong emotions such anger, laughter, surprise or anticipation. Just as night-time REM (rapid eye movement) sleep is normally accompanied by skeletal muscle paralysis and strong emotions (dreaming); an intense emotion during the waking period can trigger instantaneous muscle weakness or paralysis. Although unable to move, the person remains conscious.  For some people, any strong emotion may trigger cataplexy, while others react to only certain specific emotions. Although most patients experience cataplexy, some patients never develop this symptom.  Hypnagogic hallucinations are vivid, often frightening, dream-like images that occur when dozing or when falling asleep. Sometimes these images are so vivid that they are difficult to distinguish from reality.  Sleep paralysis refers to a temporary paralysis upon falling asleep or waking up. Episodes may last only a few seconds to minutes.   Frequent awakenings at night are common, but are not the cause of excessive daytime sleepiness in patients with narcolepsy.