The cause of restless legs is predominantly unknown, but it is thought to involve a nervous system disorder (a disorder based in the brain or spinal column). It may run in families.
Restless legs have also been related to an imbalance of dopamine in the brain. Dopamine is a brain chemical that affects movement; it naturally falls at night, which could be why Resteless Leg Syndrome symptoms often get worse late in the day and at night.
Blankets naturally supply deep pressure which may help in increasing dopamine levels.