Epidemiology of daytime sleepiness: definitions, symptomatology, and prevalence.

Daytime sleepiness, or difficulty in maintaining a desired level of wakefulness, is frequently viewed by the general population as a common experience and predictable consequence of insufficient sleep. However, daytime sleepiness can have a serious impact on an individual's health, safety, and quality of life. Despite the fact that population-based studies have found that 1 in 5 adults suffers from daytime sleepiness, there is a lack of consistency in how daytime sleepiness is defined, measured, and interpreted, which may affect the medical management of the disorder. For example, many measures of sleepiness based on sleep propensity and falling asleep tend to overlook patients with insomnia and sleepiness. Sleep scales that contain sensitive and specific questions are needed to measure fatigue and perceptions of sleepiness.