ABSTRACT. Adolescents and young adults are often excessively sleepy. This excessive sleepiness can have a profound negative effect on school performance, cognitive function, and mood and has been associated with other serious consequences such as increased incidence of automobile crashes. In this article we review available scientific knowledge about normal sleep changes in adolescents (13-22 years of age), the factors associated with chronic insufficient sleep, the effect of insufficient sleep on a variety of systems and functions, and the primary sleep disorders or organic dysfunctions that, if untreated, can cause excessive daytime sleepiness in this population. Pediatrics 2005;115:1774-1786; sleep, sleepiness, adolescents, pediatric sleep problems, daytime sleepiness, young adults, circadian rhythm, melatonin, motor vehicle accidents, depression, attention-deficit/hyperactivity disorder, school start times, school performance, obstructive sleep apnea, narcolepsy, idiopathic hypersomnia, delayed phase syndrome, insufficient sleep.