The Laboratory Adaptation begins upon admission to the laboratory (Day 1 on protocol schematic in Figure 13) and continues until Day 3. The subject will be scheduled to sleep and wake at his/her regular times (determined during the ambulatory baseline). This segment of the study is designed to allow the subject to adapt to the laboratory environment and to allow all subjects to experience a standardized lighting regime prior to initial circadian phase assessment. We believe another reason the baseline days are essential is because prior to study individual subjects will experience varying light exposure patterns and levels. The standardized lighting conditions during baseline days allow stabilization of the subjects' circadian phase and phase of entrainment, and also control immediate lighting history prior to the study interventions. Ambient lighting during waking on the baseline days will be provided by ceiling panels and will be of ordinary indoor level, and all lights will be turned off (darkness) during the scheduled sleep episodes (black boxes on double raster plot of protocol). For all scheduled sleep episodes, polysomnographic recording of sleep will be taken. Due to the nature of the study, the exact timing of the beginning and ending of each study will be based on the individual subject's habitual sleep-wake schedule. Beginning on Day 1 of the study, saliva samples will be taken hourly during waking, and blood samples will be collected twice hourly for baseline melatonin and cortisol phase/amplitude estimation. Subjective alertness assessments will be collected twice per waking hour, and neurobehavioral performance testing will be conducted every 2 waking hours.