Following the PSG screen, volunteers will then undergo four periods of study (Visits 3 through 6), each of one night and separated by at least five days, during which time participants will be administered either a single dose of prolonged-release melatonin or a matching placebo, and a single dose of temazepam, zolpidem or a matching placebo. During each of these periods, the same assessments as those conducted during the PSG screen will be repeated except that blood samples will not be drawn for melatonin assay.