Clinical and laboratory evaluations of excessive daytime sleepiness.

Sleepiness is common, dangerous, and difficult to quantitate. Sleepiness normally occurs if one tries to be awake during the circadian sleepiness phase or if one obtains insufficient sleep. Certain subjective tests of sleepiness can help the clinician to quantitate historical sleepiness. Objective tests of sleepiness are validated but imperfect predictors of future sleepiness. In spite of the difficulty in measuring sleepiness, a careful clinical evaluation of patients who complain of sleepiness is possible and usually reveals the correct diagnosis.