Patients in the ICU are known to have severely disrupted sleep with disturbed circadian pattern, decreased nocturnal sleep time, abnormally increased stages 1 and 2 sleep, and reduced or absent deep sleep. Recent data reveal that a subpopulation of critically ill patients manifests unique EEG sleep patterns. The etiology of sleep disruption in the ICU includes the inherent nature of the environment, medications, ventilator-patient interaction, and the effect of acute illness. How sleep disruption contributes to outcomes in critically ill patients, such as recovery time and weaning from mechanical ventilation, is unknown. This article reviews the literature describing sleep in ICU patients, including recent investigations in patients who require mechanical ventilation, factors that affect sleep in critically ill patients, and the potential mechanisms and clinical implications of disturbed sleep in the ICU setting with directions to consider for future investigations.