"Night owls" and "morning larks" are descriptive terms used to characterize individuals who go to sleep or awaken differently than most individuals. Many of these individuals have a primary circadian sleep dysrhythmia. Identification and proper treatment of a specific condition can markedly improve their quality of life. Secondary circadian dysrhythmias are very common. Nearly everyone at some time in his or her life experiences jet lag or shift work sleep disorder, two conditions in which we ignore our biologic rhythms. The impact of these conditions on performance and judgment can be tempered by certain pharmacologic and nonpharmacologic strategies. A better understanding of both primary and secondary circadian rhythm sleep disorders will be valuable to the primary care physician, leading to earlier diagnosis and improved treatment of patients with these conditions.