The symptoms of narcolepsy most commonly begin between the ages of 10 and 25. They may worsen for the first few years, and then continue for life. They include:

Excessive daytime sleepiness. People with narcolepsy fall asleep without warning, anywhere, anytime. For example, you may suddenly nod off while working or talking with friends. You may sleep for a few minutes or up to a half-hour before awakening and feeling refreshed, but eventually you fall asleep again.

You also may experience decreased alertness throughout the day. Excessive daytime sleepiness usually is the first symptom to appear and is often the most troublesome, making it difficult for you to concentrate and fully function.

Sudden loss of muscle tone. This condition, called cataplexy (KAT-uh-plek-see), can cause a number of physical changes, from slurred speech to complete weakness of most muscles, and may last for a few seconds to a few minutes.

Cataplexy is uncontrollable and is triggered by intense emotions, usually positive ones such as laughter or excitement, but sometimes fear, surprise or anger. For example, your head may droop uncontrollably or your knees may suddenly buckle when you laugh.

Some people with narcolepsy experience only one or two episodes of cataplexy a year, while others have numerous episodes daily. Not everyone with narcolepsy experiences cataplexy.

Sleep paralysis. People with narcolepsy often experience a temporary inability to move or speak while falling asleep or upon waking. These episodes are usually brief - lasting a few seconds or minutes - but can be frightening. You may be aware of the condition and have no difficulty recalling it afterward, even if you had no control over what was happening to you.

This sleep paralysis mimics the type of temporary paralysis that normally occurs during rapid eye movement (REM) sleep, the period of sleep during which most dreaming occurs. This temporary immobility during REM sleep may prevent your body from acting out dream activity.

Not everyone with sleep paralysis has narcolepsy, however. Many people without narcolepsy experience some episodes of sleep paralysis, especially in young adulthood.

Hallucinations. These hallucinations are called hypnagogic hallucinations if they happen as you fall asleep and hypnopompic hallucinations if they occur upon waking. They may be particularly vivid and frightening because you may be semi-awake when you begin dreaming and you experience your dreams as reality.