The term hallucination comes from the Latin alucinari, meaning "to wander in the mind." When a person sees, hears, smells, or feels something or someone that is not really there, he or she has experienced a hallucination. Although the hallucinatory state is commonly confused with that of an illusion, the latter is caused by real sense perceptions that have been misinterpreted, whether by natural phenomena or in the case of a stage illusion, by someone deliberately misdirecting and tricking an audience.
Hallucinations result when certain situations have altered one's brain metabolism from its normal level. Common causes of hallucinations are a high fever, an adverse reaction or side effect of a drug, the deliberate ingestion of a psychedelic or hallucinogenic substance (LSD, peyote, opium), an adverse reaction to alcohol, or a post-traumatic stress disorder. The grief of suffering the recent death of a loved one sometimes prompts hallucinations of hearing or seeing the relative or close friend. Those individuals experiencing psychosis or delirium are also susceptible to the manifestations of hallucinations.
While people often associate a hallucination with dramatic circumstances, sleep deprivation can prompt the phenomenon, as can boredom, fatigue, and the frightening experience of "highway hypnosis," when people have been behind the wheel driving too long and the monotony of the road causes them to see things that aren't really there.
Many individuals who suffer from migraine attacks report certain kinds of hallucinations, especially those of colored, shimmering geometric shapes, quite likely induced by changes in the retina or the visual pathway. Some researchers suggest that some of the visions experienced by certain mystics and saints were set in motion by migrainous hallucinations.
Some people have hypnopompic episodes, a kind of hallucinatory experience, while either falling asleep or waking up. They may believe that some kind of supernatural being has entered the room and settled on their chest. They may even hear the entity speaking to them in a peculiar language. Some researchers suggest that such hypnopompic hallucinations might explain the incubus and succubus phenomena of nighttime demonic attacks that have been reported since medieval times.
Hallucinations caused by sleep or sensory deprivation require no medical treatment unless the individual continues to abuse the normal bodily demands for rest. Those caused by substance and alcohol abuse may likely need medical help to allow the individual to establish normality. Hallucinatory manifestations that continue without an individual's being able to determine any physical or mental reason may require a psychiatric consultation.