Sleep disorders are a group of medical conditions that affect the quality and quantity of sleep. The most common sleep disorders are INSOMNIA, narcolepsy, and sleep apnea. INSOMNIA is a disorder characterized by difficulty falling asleep or staying asleep, resulting in inadequate sleep. Narcolepsy is a neurological disorder that causes excessive daytime sleepiness and sudden, uncontrollable episodes of sleep. Sleep apnea is a disorder in which breathing repeatedly stops and starts during sleep.