Sleep fragmentation is one of the recognized determinants of excessive daytime sleepiness. It is characterized by repetitive short interruptions of sleep. These interruptions can be variously defined using electroencephalography (EEG) autonomic or behavioral markers. An increase in the number of (short) arousals may be associated with periodic leg movements and sleep-disordered breathing. The number of arousals related to the total sleep time, the arousal index, is useful to quantify sleep fragmentation.