The most critically ill patients in the hospital are located in the ICU. Due to intensive individualized care and monitoring, these patients often suffer from severe sleep deprivation. The amount and continuity of sleep as well as normal sleep architecture are all affected. Moreover, by impairing protein synthesis, cell division, and cellular immunity, sleep deprivation can affect the healing process and thus contribute to an increased morbidity and mortality. Reasons for sleep deprivation appear to be multifactorial and include the following: the patient's chronic underlying illness, an acute superimposed illness or surgical procedure, medications used in treatment of the primary illness, and the ICU environment itself. Therapeutic interventions need to address each of these potential causes, with an emphasis placed on providing an environment that is both diurnal and focused on the importance of uninterrupted sleep.