Abnormalities of sleep are common in hospitalized patients, but the mechanisms and consequences are not well understood. In many of these patients, sleep is very disrupted, occurs during the daytime, and circadian rhythm is diminished or lost. Hospitalized patients experience more frequent arousals and awakenings than is normal and show decreases in rapid eye movement and slow wave sleep. The degree of sleep fragmentation is at least equivalent to that seen in patients with obstructive sleep apnea. About 20% of arousals and awakenings are related to noise, 10% are related to health care personnel and care-related activities, and the cause for the remainder is not known, although severity of underlying disease is likely an important factor.