Sleep paralysis occurs when a person becomes completely paralyzed when either going to sleep or waking up. The only muscles that can be moved are the eye muscles and the diaphragm. It is very frightening because the person is awake and usually thinks they are dead or have had a massive stroke. The symptoms last only a few seconds to minutes, but can recur regularly. It's associated with another sleep disorder called narcolepsy, but sleep paralysis can also occur by itself. It's a harmless condition by itself, but it's important to get evaluated and treated if it is part of narcolepsy.
A study was done recently to determine the lifetime prevalence of sleep paralysis. Researchers reviewed 35 studies on the condition and determined that almost 8% of the general population has had at least one episode. Prevalence was over 28% for students and 32% for psychiatric patients - possibly due to more disrupted sleep and changes in sleep schedules seen in those populations.