Nocturnal myoclonus, also calle Periodic Limb Movement Disorder (PLMD), is a sleep disorder where the patient moves limbs involuntarily during sleep and has symptoms or problems related to the movement. If the patient moves limbs during sleep but there are no consequences, it is simply called periodic limb movements of sleep (PLMS). Not all patients with PLMS have PLMD (in fact, most people with PLMS do not have a disorder requiring any treatment).