Patients with excessive daytime sleepiness (EDS, hypersomnia, or hypersomnolence) have impaired function due to difficulty maintaining wakefulness or alertness at appropriate times during the day. Complaints of EDS, or related terms such as tiredness, fatigue, and lack of energy, constitute some of the most common issues presented to clinicians. EDS is important to recognize because it can signal an undiagnosed sleep disorder or other treatable conditions. In addition, EDS can negatively impact a broad range of activities and have important safety considerations for activities such as driving.
This topic provides a general overview of the epidemiology, etiology, clinical features, and diagnosis of disorders that cause excessive daytime sleepiness. Classification of sleep disorders, sleep deprivation, and quantification of sleepiness are discussed separately. (See "Classification of sleep disorders" and "Definition and consequences of sleep deprivation" and "Quantifying sleepiness".)
Excessive daytime sleepiness — Daytime sleepiness is defined as excessive when it causes a subjective complaint or interferes with function. The American Academy of Sleep Medicine defines excessive daytime sleepiness (EDS) as the inability to maintain wakefulness and alertness during the major waking episodes of the day, with sleep occurring unintentionally or at inappropriate times almost daily for at least three months .
Hypersomnia — The term hypersomnia encompasses excessive sleepiness but can also refer to excessive time spent asleep. “Hypersomnias” can refer to a group of specific sleep disorders. (See 'Hypersomnias of central origin' below.)
Fatigue — Fatigue refers to a subjective lack of physical or mental energy. Clinical fatigue incorporates three components, present to variable degrees in individual patients: inability to initiate activity (perception of generalized weakness, in the absence of objective findings); reduced capacity to maintain activity (easy fatigability); and difficulty with concentration, memory, and emotional stability (mental fatigue) . (See "Approach to the adult patient with fatigue".)