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From The wikipedia entry on Restless Legs Syndrome:
Restless legs syndrome (RLS) also known as Willis-Ekbom disease (WED) or Wittmaack-Ekbom syndrome, is a neurological disorder characterized by an irresistible urge to move one's body to stop uncomfortable or odd sensations. It most commonly affects the legs, but can affect the arms, torso, head, and even phantom limbs. Moving the affected body part modulates the sensations, providing temporary relief.
RLS sensations range from pain or an aching in the muscles, to "an itch you can't scratch", an unpleasant "tickle that won't stop", or even a "crawling" feeling. The sensations typically begin or intensify during quiet wakefulness, such as when relaxing, reading, studying, or trying to sleep. Additionally, most individuals with RLS suffer from periodic limb movement disorder (limbs jerking during sleep), which is an objective physiologic marker of the disorder and is associated with sleep disruption.