Narcolepsy is a sleep disorder with daytime drowsiness, sudden episodes of falling asleep, sometimes sudden loss of muscle control, and occasionally hallucinations.
If this sounds like the bad, overbearing, uncontrolled brother of REM sleep, that is because it is. The features of REM sleep are there; sleepiness, muscle paralysis(cataplexy) and Hallucinations(dreams). This combination of symptoms can often disrupt jobs, and can be dangerous, leading to accidents.
The cause of narcolepsy is unknown, but there is an association with decreased blood levels of the neurochemical hypocretin. The disease starts in youth, and sometimes occurs and families. The patients are often overweight, and can have sleep apnea in addition.
The Diagnosis is usually made in specialist sleep centers, which find an unusually rapid entrance into sleep, beginning in the REM stage. Normally REM sleep occurs later in the 90-minute sleep cycle.
There are a lot of different stimulants and some sleep-restoring and paralysis-reducing medications used to treat narcolepsy. This and other information may be found on the accompanying Mayo clinic article.