Insufficient sleep is common, affecting 20-40% of adults, and resulting from sleep disorders, medical conditions, work demands, stress/emotional distress, and social/domestic responsibilities. It produces significant social, financial and health-related costs, and it has increasingly become a major public health concern as population studies worldwide have found that reduced sleep duration is associated with increased risks of obesity, morbidity, and mortality. It is well established that sleep loss causes fatigue and sleepiness, as well as errors and accidents that are due to its adverse neurobehavioral effects on alertness, mood, and cognitive functions. However, there are substantial, trait-like differences among people in the extent to which they experience such neurobehavioral deficits when sleep deprived. Common genetic variations involved in sleep-wake, circadian, and cognitive regulation may underlie these large inter-individual differences in neurobehavioral vulnerability to sleep deprivation, though it remains unclear whether different types of sleep deprivation involve the same phenotypic responses and same genotypic contributors. This project will be the first large-scale investigation of markers of differential cognitive vulnerability to both acute total sleep loss and chronic partial sleep loss. It will identify individuals who are at significant risk for fatigue and severe impairments from sleep loss. A total of 110 healthy adults will undergo a 13-day laboratory protocol to thoroughly characterize their cognitive, psychological and physiological responses to two of the most common forms of sleep loss--acute total sleep deprivation (1 night of sleep loss) and chronic partial sleep deprivation (5 nights of sleep limited to 4 hr). The findings from this study will represent a critical first step toward tailoring appropriate follow-up interventions for sleep loss and its symptomatic relief by finding predictors of at-risk individuals who should avoid sleep loss whenever possible, and/or seek effective countermeasures. Whether or not markers of neurobehavioral vulnerability to sleep loss are identified, the results of the project will help inform public policies pertaining to the need for adequate sleep and for countermeasures for sleep loss, and also will further our understanding and management of vulnerability to excessive sleepiness due to common sleep and medical disorders.