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Arousal accompanied by awareness of one's environment. In practice, consciousness is said to be present when a person is awake, alert, and oriented to his or her surroundings, i.e., where one is, who one is, what the date is.
Alterations of consciousness are common. Sleep is an altered state of consciousness from which one can be easily aroused. Stupor and lethargy are conditions in which one's level of arousal is diminished. In coma, one cannot be aroused. Other alterations in consciousness occur in delirium, dementia, hallucinosis, or intoxication, when persons may be fully aroused but have impaired perceptions of themselves and their environment.