Sleep-wake patterns and survival curves for the duration of every episode of sleep. (a) Representative sleep-wake patterns from one healthy control, one patient in the a.m. less sleepy group, and one patient in the a.m. sleepier group. In contrast to the control and a.m. less sleepy patient, the a.m. sleepier patient shows clustering of her arousals, which is documented in the accompanying panel. (b) Survival curves of every episode of sleep (that is, a bout of sleep preceded and followed by periods of wakefulness) for controls and patients in the a.m. less sleepy and a.m. sleepier groups for whole-night hypnograms stratified by the duration of the sleep episode. To compare sleep continuity between groups, all data from all subjects in each group were pooled and a group survival curve was generated using standard statistical techniques . Patients in the a.m. sleepier group showed a significant shift toward shorter bouts of sleep (P < 0.05) compared with the other groups. CFS, chronic fatigue syndrome.