Subjects with isolated complaints of chronic daytime sleepiness are usually classified as "idiopathic hypersomniacs" and treated symptomatically. A group of these subjects was investigated during nocturnal sleep and daytime naps. In a subgroup of them, sleep was fragmented by very short alpha EEG arousals throughout the sleeping period. These short arousals are usually ignored in sleep analyses, but their impact is significant (in the 15 subjects identified with the syndrome, the mean sleep latency in multiple sleep latency tests was 5.1 +/- 1 min). These arousals are directly related to an abnormal increase in respiratory efforts during sleep (the mean peak inspiratory esophageal pressure measured in our subjects in the respiratory cycle just preceding a transient arousal was -33 +/- 7 cm H2O). Typically, an arousal occurs within one to three breaths of flow limitation associated with abrupt but limited reduction in tidal volume (ie, abnormal increase in upper airway resistance during sleep). The arousal restores normal breathing. Snoring was noted in association with these transient arousals in 10 of the 15 subjects; however, snoring was neither sufficient nor necessary for the identification of the clinical syndrome. Both sexes were equally represented in the affected group. All studied subjects had upper airway anatomy that was mildly abnormal. Nasal continuous positive airway pressure, used as an experimental tool, eliminated the daytime sleepiness (multiple sleep latency mean score = 13.5 min), the transient arousals (mean alpha EEG arousal index decreased from 31.3 +/- 12.4 to 8 +/- 2 per hour of sleep), and the abnormal upper airway resistance. Chronic daytime sleepiness is a major cause of social, economic, and medical impairment. Recognition of this syndrome and its cause is important, as specific treatments can be developed to eliminate the problem.