Idiopathic hypersomnia (IH) includes a clinical phenotype resembling narcolepsy (with repeated, short restorative naps), and a phenotype with an excess of sleep, sleep drunkenness, drowsiness, and infrequent long, nonrestorative naps. Sleep tests reflect this heterogeneity. MSLTs are greater than 8 min in 2/3 of the cases and poorly repeatable. Sleep excess is better captured by extended monitoring identifying 11 to 16h of sleep/24 h. Patients with IH are young and more often female. Possible mechanisms of IH include deficiencies in arousal systems, inappropriate stimulation of sleep-inducing systems, and long biological night. Treatments now include robust studies of modafinil, clarithromycin, and sodium oxybate.