Sleep quality of patients in intensive care unit (ICU) has been recently recognized as an important aspect of the intensive care. Dexmedetomidine is one of the most recently introduced for sedation in the ICU. This study was designed to evaluate the effect of dexmedetomidine on sleep quality of patients without mechanical ventilation in ICU.The patients who were included in this study were divided into two groups. In the sedation group, dexmedetomidine was given by a continuous infusion targeting a sedation level –1 to –2 on the score of RASS (Richmond Agitation-Sedation Scale). In the no sedation group, the patients slept by themselves. No other sedatives were given. Bispectral Index (BIS) was performed on these hemodynamically stable critically ill patients for 12 consecutive hours. Sleep time and sleep depth were recorded.Twenty patients were studied. Compared to no sedation group, sleep efficiency and sleep time of patients in the sedation group was significantly higher during the night. Moreover, there was no significantly difference between the changes of blood pressure, heart rate, and respiratory rate.Dexmedetomidine is a clinically effective and safe sedative for the highly selected critically ill patients without endotracheal tube and mechanical ventilation in the ICU to increases total sleep time and improve sleep efficiency.