Narcolepsy is a sleep disorder that causes uncontrollable sleepiness and frequent daytime sleeping.

The exact cause of narcolepsy is unknown. Studies using gene markers show that the disorder may be genetic. A small group of neurons in the brain has been implicated in producing transitions from sleep to wakefulness and vice-versa. People with narcolepsy may have fewer of these neurons, or these neurons may have been damaged.

The disorder may be aggravated by conditions that cause insomnia, such as disruption of work schedules.

Narcolepsy is characterized by episodes of frequent, uncontrollable daytime sleeping, usually preceded by drowsiness. Sometimes, the person may start dreaming immediately upon falling asleep. The episodes usually occur after meals, but falling asleep may occur while working or driving a vehicle, having a conversation, being in any non-stimulating situation, or being in situations where there is little movement (sedentary situations).

There is a brief period of sleep, and the person awakens feeling refreshed. However, the person may again become uncontrollably sleepy a short time later.

Narcolepsy may also be associated with cataplexy, a brief episode of severe loss of muscle tone in various muscles. Generalized weakness may occur for a few moments during the transition between sleep and wakefulness (sleep paralysis). Many people with narcolepsy also have dreamlike hallucinations in the transition between sleep and wakefulness.

• Persistent drowsiness (hypersomnia -- excessive daytime sleepiness)
• Difficulty for a young adult to stay awake during classes or work
• A sudden, uncontrollable need to sleep during the day (sleep attack)
  • May occur several times during a day
  • Usually happens after eating, but may occur at any time
  • Each sleep episode lasts about 15 minutes
• Awake and refreshed feeling after each episode
• Hallucinations, visual (sight) or auditory (hearing) may come before sleep, or occur during the sleep episode
• Sudden loss of muscle tone (cataplexy)
  • May be associated with emotional reactions such as laughter or anger
  • May be similar in appearance to seizures
• Temporary inability to use muscles (sleep paralysis)
  • Occurs immediately upon awakening
  • Occurs at the onset of drowsiness

Examination and tests are used to rule out disorders that may cause similar symptoms, including sleepiness that results from seizures, sleep apnea, insomnia, restless leg syndrome, other sleep disorders, or other medical, psychiatric, or neurological (nervous system) diseases.

The diagnosis is confirmed by sleep studies (polysomnogram), particularly a study called the Multiple Sleep Latency Test (MSLT). During MSLT, the time it takes to fall asleep during daytime nap opportunities is measured .

Tests to rule out other disorders may include:

• Lab tests (variable, depending on the suspected cause)
• ECG (measures the heart's electrical activity)
• Monitoring of breathing
• EEG (brain activity measurements)
• Recording of the amount of sleep
• Sleep lab study -- recording of observed sleep cycle, including episodes of REM sleep (the part of the cycle where dreaming occurs)
• Blood tests -- determines the presence of narcolepsy-related genes, or medical illnesses associated with excessive sleepiness

Recently, patients with narcolepsy have been found to have low levels of hypocretin (a protein made by the brain) in their spinal fluid. More research will determine how useful this test is in diagnosing narcolepsy.