What leads to hypnagogic hallucinations?

Www.wikipedia.com: These hallucinations occur just before falling asleep, and affect a surprisingly high proportion of the population (in one survey 37% of the respondents experienced them twice a week [20]). The hallucinations can last from seconds to minutes, all the while the subject usually remains aware of the true nature of the images. These may be associated with narcolepsy or rarely brainstem abnomalities.