Periodic Limb Movement Disorder is ( PLMD) is repetitive cramping or jerking of the  legs. It is the only movement disorder that occurs while you are sleeping.
The movements are repetitive and rhythmic, occurring about every 20-40 seconds.

PLMD is also considered a sleep disorder, because the movements often disrupt sleep and lead to daytime sleepiness. Periodic Limb Movement Disorder can also be found is other sleep disorders, also.

PLMD is often linked with restless leg syndrome, although they are not the same. Restless leg syndrome is a condition involving strange sensations in the legs (and sometimes arms) while awake and an irresistible urge to move the limbs to relieve the sensations the difference is PLMD is only in the legs, while restless leg syndrome is or can be in the legs and the arms. About 80% of the people with restless leg syndrome also have PLMD.

PLMD can occur in any age but is usually seen in middle aged or older people.