Alertness is the state of active attention by high sensory awareness such as being watchful and prompt to meet danger or emergency, or being quick to perceive and act. It is related to psychology as well as to physiology. A lack of alertness is a symptom of a number of conditions, including narcolepsy, attention deficit disorder, chronic fatigue syndrome, depression, Addison's disease, or sleep deprivation. Pronounced lack of alertness can be graded as an altered level of consciousness. The word is formed from "alert", which comes from the Italian "all'erta" (on the watch, literally, on the height; 1618)
People who have to be alert during their jobs, such as air traffic controllers or pilots, often face challenges maintaining their alertness. Research shows that for people "...engaged in attention-intensive and monotonous tasks, retaining a constant level of alertness is rare if not impossible." If people employed in safety-related or transportation jobs have lapses in alertness, this "may lead to severe consequences in occupations ranging from air traffic control to monitoring of nuclear power plants." 
Drugs used to increase alertness
During the Second World War, US soldiers and aviators were given benzedrine, an amphetamine drug, to increase their alertness during long periods on duty. While air force pilots are able to use the drug to remain awake during combat flights, the use of amphetamines by commercial airline pilots is forbidden. British troops used 72 million amphetamine tablets in the second world war and the RAF used so many that "Methedrine won the Battle of Britain" according to one report. American bomber pilots use amphetamines ("go pills") to stay awake during long missions. The Tarnak Farm incident, in which an American F-16 pilot killed several friendly Canadian soldiers on the ground, was blamed by the pilot on his use of amphetamine. A nonjudicial hearing rejected the pilot's claim. Amphetamines are used by college and high-school students as a study and test-taking aid. Amphetamine increases energy levels, concentration, and motivation, allowing students to study for an extended period of time. These drugs are often acquired through ADHD prescriptions to students and peers, rather than illicitly produced drugs.
Vigilance is an important trait for animals in order to watch out for predators. Typically a reduction in alertness is observed for animals that live in larger groups. Studies on vigilance have been conducted on various animals including the scaly-breasted Munia.
- De Mondenard, Dr Jean-Pierre: Dopage, l'imposture des performances, Chiron, France, 2000
- Grant, D.N.W.; Air Force, UK, 1944
- Twohey, Megan (2006-03-25). "Pills become an addictive study aid". JS Online. Archived from the original on 2007-08-15. Retrieved 2007-12-02.
- The Illicit Market for ADHD Prescription Drugs in Queensland (PDF). Queensland Crime and Misconduct Commission. April 2002. Retrieved 2008-01-13.