Sleep fragmentation (an increase in the number of short EEG arousals) is considered a major determinant of excessive daytime sleepiness but is seldom quantified in sleep studies, and reference values are scarce at best. We present data on the movement arousal index (MAI) in five groups of subjects: normals, simple snorers, patients with the sleep apnea-hypopnea syndrome (SAHS), and patients with sleep-related oxygen desaturations due to chronic obstructive pulmonary disease or neuromusculoskeletal disorders. In normal subjects, the MAI was 13 +/- 7 (mean +/- SD). MAI was distinctly increased in most patients with SAHS and was strongly correlated with the apnea-hypopnea index and loss of both slow wave and REM sleep. It was corrected to normal by nasal continuous positive airway pressure (CPAP). There was some overlap of MAI between SAHS patients and snorers, suggesting that a minority of nonapneic snorers may suffer from daytime sleepiness due to upper airway dysfunction. Sleep fragmentation is not a feature of sleep-related hypoventilation due to COPD or neuromusculoskeletal disorders, and an increase of the number of movement arousals (MA) is very suggestive of upper airway dysfunction and of potential success of CPAP. Quantification of sleep fragmentation is feasible and clinically useful; it should be included in the assessment of sleep-related breathing disorders.