Delirium is a disturbance of consciousness (awareness of the person's environment) characterized by altered or shifting mental status and inattention (reduced ability to focus, sustain, or shift attention). There are also changes in cognition (basic mental functions) such as memory impairment, disorientation to time or place, and language disturbance. There also may be disturbances of perception (accurate appreciation of the environment) such as hallucinations (seeing or hearing things that are not actually there), abnormal speech, abnormal movements (including tremors or picking at clothing), disruptive or violent behavior, and sudden shifts in emotions. To qualify for a diagnosis of delirium, the signs and symptoms must have a short onset (over a period of hours or days) and change over the course of the day. It is important to make a clear distinction between delirium and dementia (such as Alzheimer disease). Dementia is a disturbance in intellectual (thinking) functions that is usually gradually progressive over a long period.