They occur at a different point of the sleep cycle from nightmares and are accompanied by considerable autonomic and motor disturbance. An illusion is a false perception of a real object or a misinterpretation of an actual stimulus, e.g. mistaking a nurse for a relative. A hallucination is a disorder of perception - a visual, auditory, tactile or olfactory sensation in the absence of a corresponding external stimulus. Hallucinations occur either in wakefulness or in the transition between sleep and wakefulness. Hypnagogic hallucinations occur when falling asleep.
Hypnopompic hallucinations occur when waking.
Both are normal and usually simple, such as hearing a telephone or doorbell or hearing one's name being called. Drugs may interfere with sleep patterns. Anticholinergic drugs, tricyclic antidepressants, alcohol, caffeine and hypnotics may contribute to nightmares. Withdrawal of alcohol or sedatives may do likewise.