Both disorders affect the limbs, and a person's ability to sleep at night and function normally during the daytime, but they are different disorders. Restless Leg Syndrome (RLS) occurs while a person is awake, as a constant need to stretch or move the legs to get rid of uncomfortable feelings in the legs. This also carries over into sleep time, resulting in disrupted patterns of sleep, and daytime sleepiness. Periodic Limb Movement Disorder (PLMD) occurs most often when a person is asleep, and involves disruptive movements of the legs and sometimes the arms. It, too, results in a disrupted pattern of sleep and consequently daytime sleepiness. Both disorders can be helped with medications.