What can cause hypnagogic hallucinations?

Hypnogogic : Hypnogogic hallucinations are visual, tactile, auditory, or other sensory events that occur at the transition from wakefulness to sleep. They can be part of the syndrome of narcolepsy. Narcolepsy is a sleep disorder consisting of the above, along with daytime sleepiness, cataplexy (suddenly going limp), and sleep paralysis. Other possibilities include partial and absence seizures, & schizophrenia.