The initial Constant Posture (CP) circadian phase estimation begins eight hours before habitual sleep time on Day 3 and continues for 22 h (shown in the cross-hatched bars in Figure 13). Throughout the CP, the subject will be restricted to a semi-recumbent position in bed in very dim indoor light (see Lighting Conditions section below for description) and will be fed equivalent hourly snacks. This segment of the study is designed to allow assessment of circadian phase by either eliminating factors known or suspected to obscure or mask circadian rhythms (changes in posture, activity, food intake, light level) or by distributing such factors across all circadian phases. The subject will be attended by a trained staff member to ensure wakefulness throughout the CP. Collection of plasma melatonin and cortisol, subjective alertness and neurobehavioral performance data will continue as in the Laboratory Adaptation Segment. After the CP is complete, the subject will be allowed to be ambulatory for another 6 hours before an 8-hour sleep episode. We chose to use CPs rather than a Constant Routine in order to avoid sleep deprivation, the recovery from which would obscure the results of the treatment conditions.