Daytime somnolence. Basic concepts, assessment tools and clinical applications.

Excessive somnolence is a common symptom, with a prevalence of 10 to 20% in a general population. However, physicians seldom ask their patients about sleep complaints. The internal biological clock drives the balance between sleepiness and alertness, generating circadian rhythms, with "physiological" increases of somnolence, especially at mid-day and before the habitual bed time. Excessive somnolence is a subjective feeling of an imperious need of sleep in unusual time and environmental conditions. Sleep deprivation, sleep fragmentation and to a lesser degree hypoxia are believed to be the main mechanisms leading to excessive somnolence. Excessive somnolence increases the risk of car accidents, deteriorates health status and quality of life and might increase mortality. Excessive somnolence is associated with many diseases such as obstructive sleep apnoea syndrome. Excessive sleepiness can be assessed by visual scales or questionnaires, the best known being the Epworth sleepiness scale. Objective tests in somnolent patients assess the sleep-wake balance disturbances. The most widely used tests are the multiple sleep latency test (MSLT), the maintenance of wakefulness test (MWT) and the Oxford sleep resistance (OSLER) test. These tests measure the time to sleep emergence in different conditions.