INTRODUCTION
Patients with excessive daytime sleepiness (EDS) 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 condition. In addition, EDS can have negative impacts on a broad range of activities and raise safety risks while driving or operating other machinery.
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 "Insufficient sleep: Definition, epidemiology, and adverse outcomes" and "Quantifying sleepiness".)
DEFINITIONS
Excessive daytime sleepiness — Daytime sleepiness is defined as excessive when it causes a subjective complaint or interferes with function. The International Classification of Sleep Disorders, Third Edition, Text Revision (ICSD-3-TR) defines 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 [1].
Hypersomnia — The terms hypersomnia and hypersomnolence are sometimes used interchangeably with EDS. In the ICSD-3-TR, hypersomnolence is used to describe symptoms including excessive sleepiness and increased sleep duration, and hypersomnia refers to specific disorders characterized by hypersomnolence [1]. (See 'Central disorders of hypersomnolence' 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) [2]. (See "Approach to the adult patient with fatigue".)
