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.