This study uses a randomized, double-blind, placebo-controlled, parallel groups design. The independent variable will be melatonin treatment group (0.3 mg melatonin, 3.0 mg melatonin, or placebo). Day of treatment will be an independent variable in analysis of sleep, cortisol, performance and alertness data. The primary outcome variables will be sleep efficiency and the peak of the plasma cortisol rhythm. Subjective and objective measures of alertness and performance during wake time will also be assessed as part of an exploratory analysis. Safety of short-term melatonin treatment will be assessed. The experimental protocol is divided into 5 segments, the ambulatory baseline segment, the laboratory adaptation segment, the initial constant posture (CP, to assess circadian phase), the melatonin/placebo intervention before advanced scheduled sleep, and second CP to assess final circadian phase.

The experimental protocol is divided into 5 segments, the ambulatory baseline segment, the laboratory adaptation segment, the initial constant posture (CP) for circadian phase assessment, the melatonin/placebo intervention, and a final post-treatment CP for assessment of circadian phase.

The Ambulatory Baseline consists of at least 21 days, outside the laboratory. During this segment, wrist activity and light levels will be recorded using an ambulatory recording device (Actiwatch-L) while the subject goes about his/her normal routine. During this segment, the subject will maintain a regular, self-selected sleep-wake/light-dark schedule (8h in bed, 16h awake), and a sleep diary. The subject will also call in to our time-stamped sleep-wake call-in telephone system just prior to each bedtime and immediately after each wake time.

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.

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.

The melatonin/placebo intervention days follow the initial CP. On experimental Day 4-5, scheduled sleep will occur 5 hours before habitual bedtime. Melatonin or placebo will be administered 30 minutes before lights-out. Twelve hours after waking the following day (Day 5), the CP to asses final circadian phase will begin and continue for 22 hours. The final sleep episode will be scheduled from 18:00 to 02:00 h to ensure that post-treatment phase estimates are in no way compromised by change of sleep time. Subjects will be informed that they are likely to experience sleep disruption, impaired alertness and other symptoms of jetlag for up to several days after they are discharged from the laboratory. Four hours after the CP on Day 7, the subject will be discharged home.