Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Narcolepsy ?
    ### Response:
    Narcolepsy (NAR-ko-lep-se) is a disorder that causes periods of extreme daytime sleepiness. The disorder also may cause muscle weakness.
                
Most people who have narcolepsy have trouble sleeping at night. Some people who have the disorder fall asleep suddenly, even if they're in the middle of talking, eating, or another activity.
    