(HealthDay News) -- Narcolepsy is a nervous system disorder characterized by sudden daytime sleep attacks, and being excessively sleepy. The disorder tends to affect more than one family member.

The U.S. National Library of Medicine says warning signs of narcolepsy may include:

• Periods of overwhelming drowsiness during the day, about every three to four hours.
• Feeling a strong need to sleep, then suddenly falling asleep.
• Attacks of sleep and drowsiness that last about 15 minutes. Afterward, the person feels quite refreshed.
• Having dreams or hallucinations that occur between sleeping and waking.
• An inability to move while awake for a while, or as soon as the person wakes.