Exclusionary criteria for all subjects will include: evidence of a clinically significant sleep disorder that would cause hypersomnolence (e.g. moderate to severe obstructive sleep apnea, restless legs syndrome, shift-work sleep disorder), history of significant head trauma or loss of consciousness > 30 minutes; current smoking of more than 15 cigarettes per day; >3 caffeinated beverages per day; significant neurologic or medical illness; active drug/alcohol abuse/dependence (within 6 months of enrollment), women who are pregnant, <6 months post-partum, nursing or planning to become pregnant during the study; left-handedness (due to effects on sleep topography); and imminent risk for self-harm or suicide.