Below is an instruction from Human. Write a response.
    ### Instruction:
    What causes Narcolepsy ?
    ### Response:
    Most people who have narcolepsy have low levels of hypocretin. This is a chemical in the brain that helps promote wakefulness. What causes low hypocretin levels isn't well understood.
                
Researchers think that certain factors may work together to cause a lack of hypocretin.
    