Narcolepsy is a chronic neurological disorder characterized by sudden overpowering attacks of sleep episodes throughout the day. People with narcolepsy experience extreme daytime sleepiness, and can suddenly fall asleep any time regardless of time and place. Narcolepsy can lead to serious disturbances in daily activities, such as reluctantly falling asleep at work, while talking, eating or even driving, thereby greatly affecting the concentration, alertness and functionality of the affected individual. A typical night of sleep normally consists of 4-6 sleep-wake cycles for adults. Each sleep cycle involves two phases: non-rapid eye movement (NREM) sleep followed by rapid eye movement (REM) sleep. Upon falling asleep, the NREM phase is first entered in the early stages, followed by deeper sleep stages depending on the size and frequency of brain waves. The transition from NREM to REM sleep usually occurs after ~90min of sleep for normal people. REM sleep involved bursts of rapid eye movement with occurrence of dreams and muscle paralysis. During REM, muscles become temporarily limp, as it prevents people from acting out the dreams. The normal sleep-wake cycle is disrupted in people with narcolepsy. They enter into REM sleep within the first few minutes of the sleep cycle and wake up from it directly. Therefore, they often experience vivid dreams and muscle paralysis upon waking up. References: Dauvilliers Y, Arnulf I, Mignot E. (2007). Narcolepsy with cataplexy. Lancet. 369(9560): 499-511.