This is a single center randomized pilot study comparing the effects of an α2 agonist (dexmedetomidine) versus GABA agonists (propofol or a benzodiazepine) on total sleep time and sleep quality. For the purposes of enrollment and analysis all benzodiazepines used for sedation (mainly midazolam and lorazepam) will be considered equivalent. Patients who are mechanically ventilated and sedated will be enrolled. The initial sedative will be determined by the managing medical team and the medication will be active at the time of enrollment. The patients will then be randomized to either continue their current sedative or be switched to either propofol or dexmedetomidine. PSG data will be collected for up to 96 hours, beginning at enrollment for all patients. The analysis of PSG will not begin until after a 8 hour "washout" period has completed to minimize carryover effect of prior sedatives.