The importance of sleep quality for health has been reported yet remains underappreciated by both health care professionals and the general public. Several lines of evidence indicate that sleep quality may be a marker of overall health. Epidemiologic surveys show an association between shortened sleep duration and obesity, cardiovascular disease, and diabetes. Physiological studies indicate that short-term sleep loss results in alterations in metabolic and immune function. Survey data show that medical disorders are often associated with self-reported poor sleep. Patients with chronic pain (arthritis, fibromyalgia) and gastrointestinal (gastroesophageal reflux disease), cardiovascular (coronary heart disease, congestive heart failure, hypertension), pulmonary (chronic obstructive pulmonary disease, asthma), and metabolic disorders (obesity, diabetes) are at increased risk for disturbed sleep. Increasing evidence points to a bidirectional relationship between sleep and health, so that sleep disturbances contribute to the development of or increase in the severity of various medical disorders; these same disorders result in poor sleep quality. Still, little is known about the mechanisms for these relationships and whether improving sleep can modify the course of comorbid medical disorders.