Excessive daytime sleepiness (EDS) is often observed in obstructive sleep apnea (OSA), although many subjects do not report the symptom. EDS is associated with increased healthcare utilization and could be one of the main determinants of an increased accident risk. Its possible role as a marker of cardiometabolic risk is debated. The pathogenetic mechanisms of EDS include sleep fragmentation, nocturnal intermittent hypoxemia, increased respiratory efforts, altered sympathovagal balance, depression, inflammatory, and metabolic alterations. Subjective EDS is uncommon when OSA is diagnosed after stroke or cardiac failure, and is a more prominent symptom in young than in elderly people. Insomnia, instead of EDS, may occur in OSA, particularly among women and in subjects with mild nocturnal respiratory disorders. OSA treatment usually greatly improves EDS, but it may not be effective in about 6 % subjects.