timated that about 1% of the European and U.S. general population has episodes of sleep
inertia. Hypersomnolence occurs with relati vely equal frequency in males and females.
Development and Course
Hypersomnolence disorder has a persistent course, with a prog ressive evolution in the se-
verity of symptoms. In most extreme cases, sl eep episodes can last up to 20 hours. How-
ever, the average nighttime sleep duration is  around 9½ hours. While many individuals
with hypersomnolence are able to reduce their sleep time during working days, weekend
and holiday sleep is greatly increased (by up to 3 hours). Awakenings are very difficult
and accompanied by sleep inertia episodes in nearly 40% of cases. Hypersomnolence fully
manifests in most cases in late adolescence or early adulthood, with a mean age at onset of
17–24 years. Individuals with hypersomnolence disorder are diagnosed, on average, 10–15
years after the appearance of the firs t symptoms. Pediatric cases are rare. 
Hypersomnolence has a progressive onset, wi th symptoms beginning between ages 15
and 25 years, with a gradual progression over weeks to months. For most individuals, the
course is then persistent and stable, unless tr eatment is initiated. The development of other
sleep disorders (e.g., breathing-related sleep disorder) may worsen the degree of sleepi-
ness. Although hyperactivity may be one of th e presenting signs of daytime sleepiness in
children, voluntary napping increases with ag e. This normal phenomenon is distinct from
hypersomnolence.
Risk and Prognostic Factors
Environmental. Hypersomnolence can be increased te mporarily by psychological stress