 Over 1 million patients receive mechanical ventilation every year in the United States. In mechanically ventilated patients, despite sedative infusions, sleep is severely disrupted. Sleep disruption, in turn, can lead to activation of sympathetic nervous system and elevated inflammatory cytokines. Both sympathetic hyperactivity and elevated cytokines have been associated with delirium, which, in turn, is associated with increased mortality and higher healthcare costs. Currently, however, there is very little understanding of the inter-relationship between critical illness, sleep, and neuropsychological well-being. The purpose of this study is to collect preliminary data on the short-term effects of sedation with and without sympatholysis on sleep and inflammation in critically ill patients receiving mechanical ventilation. This pilot study will be accomplished by performing sleep studies and circulating cytokine measurements in critically ill patients randomized to receive two different sedation strategies: Dexmedetomidine versus Midazolam and Fentanyl. The ultimate goal of this program of research is to identify sedation practices that are least associated with adverse short- and long-term consequences, and thereby ultimately help improve sleep quality and quality of life in patients surviving critical illness. 