The main characteristic of narcolepsy is excessive and overwhelming daytime sleepiness, even after adequate nighttime sleep. A person with narcolepsy is likely to become drowsy or fall asleep, often at inappropriate times and places. Daytime sleep attacks may occur with or without warning and may be irresistible. These attacks can occur repeatedly in a single day. Drowsiness may persist for prolonged periods of time. In addition, nighttime sleep may be fragmented with frequent awakenings.