If it is an occasional occurrence, it can happen in normal
people too. If it persists it could be due to an entity called
sleep paralysis. When this happens in sleep the person has difficulty moving his hands or feet. This is the symptom of a sleep disorder called Narcolepsy. This sleep disorder is characterized by excessive daytime sleepiness, sleep paralysis, cataplexy where the person has episodes of loss of muscle function while awake, hypnagogic hallucinations and automatic behavior.