Concepts of fatigue, sleepiness, and alertness

The terms fatigue and sleepiness are often used synonymously. Both may be associated with impaired alertness; however, their underlying causes and measurement are distinct. Objective and subjective measurements of sleepiness have been well validated; however, objective measurements of fatigue are lacking and currently there is no «gold standard.» Alertness can be affected by a variety of medical and psychiatric conditions, as well as physiologic factors like the need for rest or sleep. Countermeasures to improve alertness include pharmacological as well as behavioral interventions and may depend on whether deficits in alertness are associated with fatigue or sleepiness. Ultimately sleep or rest is needed for reversal of sleepiness or fatigue.