They may have what are called hypnagogic hallucinations. Hypnagogic means as I fall asleep, on the other hand, there can be hypnopompic, which means as I wake up, so hallucinations can occur while one falls asleep. If one has narcolepsy or when one wakes up falling asleep, hallucinations are more common. They're vivid, strange nightmares. They tend to be very frightening. Again, they can last for just a few moments or minutes, and they're not dangerous, but they're very frightening and scary for the person.