Hallucinations occur when sensations are perceived in the absence of environmental stimuli. They are generated by the brain under normal or abnormal situations, including drowsiness, sensory deprivation, use of or withdrawal from drugs or toxins, structural or metabolic brain disease, seizures or migraine, and psychiatric disorders such as schizophrenia. Hypnagogic and hypnopompic hallucinations (HH) are typically visual, but can be auditory, tactile or kinetic. Complex nocturnal visual hallucinations (CNVH) have somewhat different phenomenology and putative pathophysiology from HHs and can be seen in a variety of pathologic conditions. CNVH have similar phenomenology and represent a final common pathway for a variety of etiologies. The exploding head syndrome (EHS) is thought to be a benign condition characterized by an imagined very loud sound or explosion in the head at sleep onset or on waking during night.