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Alertness is a state of active attention characterized by high sensory awareness. Someone who is alert is vigilant and promptly meets danger or emergency, or is quick to perceive and act. Alertness is a psychological and physiological state.
Lack of alertness is a symptom of a number of conditions, including narcolepsy, attention deficit disorder, chronic fatigue syndrome, depression, Addison's disease, and sleep deprivation. Pronounced lack of alertness is an altered level of consciousness. States with low levels of alertness include drowsiness.
The word is formed from "alert", which comes from the Italian all'erta (on the watch, literally: on the height; 1618).[citation needed]
Wakefulness refers mainly to differences between the sleep and waking states; vigilance refers to sustained alertness and concentration. Both terms are sometimes used synonymously with alertness.