Sleep is a physiologic state that performs an essential restorative function and facilitates learning and memory consolidation. When sleep is disrupted for more than a short time, normal daily functions decline. Mood, attention, and behavior deteriorate. Sleepiness and disrupted sleep can result from a large number of pathological disorders. Currently, 88 sleep disorders are listed in the International Classification of Sleep Disorders, as established by the American Academy of Sleep Medicine, and sleep disorders adversely affect more than an estimated 70 million Americans. Most of these disorders can be classified as causing insomnia and/or hypersomnia. Insomnia results from disorders that cause difficulty with falling asleep and staying asleep; examples are hyperarousal, circadian dysrhythmia, and homeostatic dysregulation. In contrast, hypersomnia refers to difficulty in staying awake and is characterized by recurrent episodes of excessive daytime sleepiness or prolonged nighttime sleep. Hypersomnia can result from several primary sleep disorders, including narcolepsy, sleep apnea, restless legs syndrome, idiopathic hypersomnia, and periodic limb movement disorder. The effects of some of these sleep disorders and other chronic illnesses on daytime sleepiness are measured using the Epworth Sleepiness Scale. Narcolepsy was found to cause some of the highest measures of excessive sleepiness. This supplement uses a case-based approach to describe the underlying pathology and symptoms of narcolepsy. Differential diagnosis of narcolepsy and current treatment options will be discussed.