Underlying health condition
Restless legs syndrome can sometimes occur as a complication of another health condition, or it can be the result of another health-related factor. This is known as secondary restless legs syndrome. You can develop secondary restless legs syndrome if you:
-- have iron deficiency anaemia – low levels of iron in the blood can lead to a fall in dopamine, triggering restless legs syndrome
-- have a long-term health condition – such as chronic kidney disease, diabetes, Parkinson's disease, rheumatoid arthritis, fibromyalgia 
-- are pregnant – particularly from week 27 until birth; in most cases the symptoms disappear within four weeks of giving birth

Dopamine
There's evidence to suggest restless legs syndrome is related to a problem with part of the brain called the basal ganglia. The basal ganglia uses a chemical (neurotransmitter) called dopamine to help control muscle activity and movement. Dopamine acts as a messenger between the brain and nervous system to help the brain regulate and co-ordinate movement. If nerve cells become damaged, the amount of dopamine in the brain is reduced, which causes muscle spasms and involuntary movements. Dopamine levels naturally fall towards the end of the day, which may explain why the symptoms of restless legs syndrome are often worse in the evening and during the night.

Triggers
There are a number of triggers that don't cause restless legs syndrome, but can make symptoms worse. These include medications such as:
-- some antihistamines
-- metoclopramide – used to relieve nausea
-- some antidepressants
-- antipsychotics
-- lithium – used in the treatment of bipolar disorder
-- calcium channel blockers – used in the treatment of high blood pressure