Narcolepsy:
Human narcolepsy manifests with instability of wake, sleep, and REM-sleep states.
One of the brain neuronal systems, orexin/hypocretin system, is associated with awake status and probably with stability of wake and sleep.
In narcolepsy a dysfunction of this system causes wake state to be unstable. People with narcolepsy have sleepiness and may fall asleep unexpectedly.
People with narcolepsy have unstable sleep as well. They wake up frequently and have a fragmented sleep.
REM-sleep is unstable as well.
REM-sleep has two components, it is associated with dreaming and an adult human is almost paralyzed in REM-sleep. (eye muscles and respiratory muscles are not paralyzed)
In narcoleptics, the components of REM-sleep may not occur at proper time. The paralysis of REM may occur immediately after waking up. This is called sleep paralysis. The paralysis may start to happen with emotions, like laughter. This is called cataplexy. Also dreaming may happen immediately after awakening or while falling asleep. These are called hypnagogic or hypnapompic hallucinations.
Narcolepsy is not a life threatening or dangerous condition. The main problem is the risk of car accidents. People with narcolepsy should not be involved with certain professions, including flying or operating machinary.
Narcolepsy can be managed by medications.