This study will enroll healthy individuals. Participants will first attend two screening visits, which will include a review of medical, psychiatric, and sleep histories; vital sign measurements; blood and urine collection; a physical examination; and an electrocardiogram. For 1 week, participants will record sleep habits in a diary and by telephone. They will also wear a device that monitors activity and light exposure levels. Eligible participants will then attend four 5-day inpatient visits at the Sleep and Chronobiology Laboratory at the University of Colorado at Boulder. During each of the four visits, participants will be randomly assigned to one of the following four treatments: