Unlike Restless Leg Syndrome, which occurs while the patient is awake. Periodic Limb Movement Disorder (PLMD) is a sleep disorder characterized by rhythmic movements of the limbs while the patient is asleep. The movements typically involve the legs, but upper extremity movements may also occur. Movements occur periodically throughout the night and can fluctuate in severity from one night to the next. They tend to cluster in episodes that last anywhere from a few minutes to several hours. These movements are very different from the normal spasms that we often experience initially while trying to fall asleep. The causes of PLMD are unknown. However recent research has shown that people with a variety of medical problems, including Parkinson's disease and narcolepsy, may have frequent periodic limb movements in sleep. PLMD may be induced by medications, most notably, antidepressants. Periodic Limb Movement Disorder typically results in fragmented and unrefreshed sleep. A patient may occasionally have both Periodic Limb Movement Disorder and Restless Leg Syndrome.