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Excessive daytime sleepiness (EDS) is a common problem affecting large segments of the general population. Although estimates depend on how sleepiness is defined (i.e., sleeping too much vs. falling asleep in the daytime), the National Sleep Foundation’s annual Sleep in America poll in 2018 showed that among U.S. adults with excellent sleep health, 90% say that they feel effective at getting things done, while only 46% with poor health say they can get things done effectively; only 10% of American adults prioritize sleep over other daily activities like fitness/nutrition, work, social life, and hobbies.1

There is increasing evidence that sleepiness plays a part in both industrial and road traffic accidents and impaired performance. Sleepiness is most often caused by insufficient sleep or sleep disorders.


Sleepiness is both a subjective and an objective phenomenon, a constellation of sensations and a physiologic state with stereotypical behaviors. As such, it is sometimes difficult to define, and its measurement (see “Quantifying Sleepiness,” below) depends on the circumstances. Sleepiness may be expressed as feeling sleepy, fatigued, or tired; sleeping too much; falling asleep inappropriately; or fighting to maintain alertness. Sleepiness can be reflected by any or all the following: heaviness of the eyelids, mild burning or itching of the eyes, difficulty keeping the eyes open, heaviness in the arms or legs, reluctance to move, loss of initiative, loss of interest in surroundings, and difficulty with concentration or memory. These sensations are accompanied by behavioral changes, such as rubbing the eyes, yawning and stretching, and nodding the head, and by generally reduced motor functions, such as speech, facial expression, and body movement. Sleep-deprived healthy individuals demonstrate attenuated facial expressions to stimuli.2

Sleepiness may also be considered a physiologic state like hunger or thirst. Just as hunger and thirst are physiologic states that occur with fasting and are satisfied by eating and drinking, sleepiness in individuals without a specific sleep disorder is produced by sleep restriction or deprivation and is reversed or satisfied by sleep. However, a difference between sleep and other physiologic factors is that, for example, while food may satisfy hunger, sleep quality and quantity are necessary to satiate sleep loss.

Sleep quality is impaired in individuals with specific sleep disorders, such as sleep apnea. The factors that produce and influence sleepiness are detailed below; they include such obvious factors as time since last asleep, previous amount of sleep, continuity of sleep, and normal 24-h circadian influences. Environmental stimuli influence this state and can determine, up to a point, whether this sleepy tendency will be manifested. For example, heavy meals, warm rooms, boring lectures, or monotonous tasks are usually considered soporific activities or situations. Under these circumstances, a person might feel sleepy and might fall asleep. However, the environmental factors themselves do not cause the sleepiness; they only allow it to ...

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