The initial chapter is an overview of sleep disorders. A 1995 Gallup Poll commissioned by the National Sleep Foundation (R1558271A) revealed that 40% of adults in the United States have occasional or chronic insomnia. There is a great cost to the economy in terms of decreased efficiency and personal stress due to sleep loss. The first chapter also reviews the various classification systems, including DSM-IV and the International Classification of Sleep Disorders of the American Sleep Disorders association. In order to begin classifying a sleep complaint, three questions are proposed as a good initial screen. First, the question, "Are you satisfied with your sleep?" will pick up most insomnias. Then asking, "Are you excessively sleepy during the day?" will reveal most excessive sleep disorders. Finally, parasomnias can be uncovered by asking, "Does your bed partner complain about your sleep?" These unusual events occurring during sleep are more often reported by observers of the patient's sleep—the bed partner, roommate, or parent. As with much of medicine, a good history will make most diagnoses, aided by sleep laboratory tests, such as polysomnography and the Multiple Sleep Latency Test. The second chapter reviews the physiology and pathology of sleep stages, circadian rhythms, and dreams.