What are hypnagogic hallucinations?

Hypnogogic : Hypnogogic hallucinations are visual, tactile, auditory, or other sensory events -- usually brief but occasionally prolonged -- that occur at the transition from wakefulness to sleep. They can come with the syndrome of narcolepsy, but they don't have to. Narcolepsy is a sleep disorder consisting of the above, along with daytime sleepiness, cataplexy (suddenly going limp), and sleep paralysis.