Narcolepsy is a long-term neurological disorder that involves a decreased ability to regulate sleep–wake cycles. Symptoms often include periods of excessive daytime sleepiness and brief involuntary sleep episodes. About 70% of those affected also experience episodes of sudden loss of muscle strength, known as cataplexy. Narcolepsy paired with cataplexy is evidenced to be an autoimmune disorder. These experiences of cataplexy can be brought on by strong emotions. Less commonly, there may be vivid hallucinations or an inability to move (sleep paralysis) while falling asleep or waking up. People with narcolepsy tend to sleep about the same number of hours per day as people without, but the quality of sleep tends to be lessened.Narcolepsy is a clinical syndrome of hypothalamic disorder, however, the exact cause of narcolepsy is unknown, with potentially several causes. In up to 10% of cases, there is a family history of the disorder. Often, those affected have low levels of the neuropeptide orexin, which may be due to an autoimmune disorder triggered in genetically susceptible individuals by infection with H1N1 influenza. In rare cases, narcolepsy can be caused by traumatic brain injury, tumors, or other diseases affecting the parts of the brain that regulate wakefulness or REM sleep. Diagnosis is typically based on the symptoms and sleep studies, after ruling out other potential causes. Excessive daytime sleepiness can also be caused by other sleep disorders such as sleep apnea, major depressive disorder, anemia, heart failure, drinking alcohol and not getting enough sleep.
In addition, the hypersomnolence and sleep latency findings cannot be better explained by other causes.DSM-5 narcolepsy criteria requires that the person to display recurrent periods of "an irrepressible need to sleep, lapsing into sleep, or napping" for at least three times a week over a period of three months. The individual must also display one of the following: cataplexy, hypocretin-1 concentration of less than 110 pg/mL, REM sleep latency of less than 15 minutes, or a multiple sleep latency test (MSLT) showing sleep latency of less than 8 minutes and two or more SOREMPs. For a diagnosis of hypersomnolence disorder, the individual must present with excessive sleepiness despite at least 7 hours of sleep as well as either recurrent lapses into daytime sleep, nonrestorative sleep episodes of 9 or more hours, or difficulty staying awake after awakening. In addition, the hypersomnolence must occur at least three times a week for a period of three months, and must be accompanied by significant distress or impairment. It also cannot be explained by another sleep disorder, coexisting mental or medical disorders, or medication. Tests
Diagnosis is relatively easy when all the symptoms of narcolepsy are present, but if the sleep attacks are isolated and cataplexy is mild or absent, diagnosis is more difficult. Three tests that are commonly used in diagnosing narcolepsy are polysomnography (PSG), the multiple sleep latency test (MSLT), and the Epworth Sleepiness Scale (ESS).