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Periodic limb movement disorder (PLMD) is a type of sleep disorder. While asleep, patients with the disorder unconsciously flex joints in the legs and less frequently in the arms. This can cause patients to repeatedly awaken, and lead to insomnia and/or daytime tiredness.

PLMD was originally named nocturnal myoclonus, and was thought to be related to epilepsy. It was later renamed PLMD because the movements are slower than those related to myoclonus (a condition characterized by jerky, involuntary muscle contractions). PLMD differs from other movement disorders in that it occurs only during sleep. PLMD is also sometimes known as periodic leg movement while sleeping (PLMS). When it causes a patient to awaken from sleep, it is sometimes referred to as periodic leg movement while waking (PLMW).

The prevalence of PLMD is uncertain, although the National Sleep Foundation estimates that 35 percent of adults 65 and older experience the condition. PLMD appears to affect men and women equally.

PLMD may occur on its own or in conjunction with other conditions. It is often associated with restless leg syndrome (RLS), which produces irritating sensations in the legs while resting or sleeping. It is estimated that more than 80 percent of patients with RLS also have PLMD, according to the National Institute of Neurological Disorders and Stroke (NINDS). However, the reverse is not true. Patients with PLMD are not necessarily more prone to experiencing RLS. According to the American Association for Respiratory Care, only 30 percent of patients with PLMD also have RLS.

PMLD has been linked to abnormalities in the brain stem, which regulates consciousness and arousal. It may also result from changes in levels of dopamine, a neurotransmitter that is typically involved in regulating balance and movement.