from http://www.ninds.nih.gov/disorders/narcolepsy/detail_narcolepsy.htm


What is Narcolepsy?


Narcolepsy is a chronic disorder of the central nervous system characterized by the brain's inability to control sleep-wake cycles. At various times throughout the day, people with narcolepsy experience irresistible and sudden bouts of sleep, which can last from a few seconds to several minutes.

In narcolepsy, sleep episodes can occur at any time. People may unwillingly fall asleep while at work or at school, when having a conversation, playing a game, eating a meal, or, most dangerously, when driving an automobile or operating other types of machinery. In addition to daytime sleepiness, other major symptoms include cataplexy (a sudden loss of voluntary muscle tone that may be triggered by strong emotions), vivid dream-like images or hallucinations during sleep onset or when waking, and brief episodes of total paralysis, also during sleep onset or when waking. The loss of muscle tone in cataplexy and sleep paralysis involves the simultaneous loss of both extensor reflexes (such as a knee tap and resulting leg jerk) and flexor reflexes (such as lifting the foot and/or leg following a foot prick or stepping on a sharp object). Normally, this kind of reflex loss exists only during REM sleep.

Contrary to common beliefs, people with narcolepsy do not spend a substantially greater proportion of their time asleep during a 24-hour period than do normal sleepers. In addition to daytime drowsiness and uncontrollable sleep episodes, most individuals also experience poor sleep quality that can involve frequent awakenings during nighttime sleep, and other sleep disorders. For these reasons, narcolepsy is considered to be a disorder involving the loss of control of the normal boundaries between the sleeping and waking states.

For most adults, a normal night's sleep lasts about 8 hours and is composed of four to six separate sleep cycles. A sleep cycle is defined by a segment of non-rapid eye movement (NREM) sleep followed by a period of rapid eye movement (REM) sleep. The NREM segment can be further divided into increasingly deeper stages of sleep according to the size and frequency of brain waves. REM sleep is accompanied by bursts of rapid eye movement along with sharply heightened brain activity and temporary paralysis of the muscles that control posture and body movement. When subjects are awakened, they report that they were "having a dream" more often if they had been in REM sleep than if they had been in NREM sleep. Transitions from NREM to REM sleep are controlled by interactions among groups of neurons (nerve cells) located in different parts of the brain.

Narcolepsy may have several causes. When cataplexy is present, narcolepsy is almost always caused by the lack of a brain neurotransmitter called hypocretin. Neurotransmitters are chemicals that neurons produce to communicate with each other and to regulate biological processes. Disruption typically arises from the death of approximately 70,000 brain cells containing hypocretin. Loss of hypocretin then results in an inability to regulate sleep. The normal organization of sleep is also changed. For normal sleepers a typical sleep cycle is about 100 to 110 minutes long, beginning with NREM sleep and transitioning to REM sleep after 80 to 100 minutes. People with narcolepsy frequently enter REM sleep within a few minutes of falling asleep. In cases without cataplexy, the disorder may be caused by a lack of hypocretin or by various other causes.

More info here....http://www.ninds.nih.gov/disorders/narcolepsy/detail_narcolepsy.htm