Restless leg syndrome (RLS) is driven by a combination of low brain iron levels, disrupted dopamine signaling, and genetic susceptibility. About 7% of adults worldwide are affected, which translates to roughly 356 million people. While the condition has no single cause, researchers have identified several overlapping mechanisms that explain why the urge to move your legs strikes when it does.
Iron Deficiency in the Brain
The most well-supported explanation for RLS centers on iron, but not necessarily the kind that shows up on a standard blood test. People with RLS frequently have reduced iron levels in specific brain regions, particularly an area called the substantia nigra, which plays a key role in movement control. Studies examining spinal fluid have found that RLS patients have significantly lower ferritin (the body’s iron storage protein) and higher transferrin (the protein that carries iron) compared to controls, pointing to impaired iron transport across the blood-brain barrier.
This matters because iron is essential for producing dopamine, the chemical messenger most directly involved in RLS. Without adequate brain iron, dopamine production falters and the receptors that respond to dopamine don’t function properly. Research on iron-deficient animals shows they develop the same receptor changes seen in RLS patients: fewer dopamine receptors and reduced dopamine transporters in movement-related brain areas.
What’s tricky is that your blood iron levels can appear completely normal while your brain is still iron-deficient. One study found that hemoglobin and serum ferritin levels were within the normal range for both RLS patients and healthy controls, yet the RLS group had significantly lower iron markers in their spinal fluid. Current clinical guidelines flag serum ferritin below 45 micrograms per liter as a useful threshold in older adults, and treatment aims to raise ferritin above 200 micrograms per liter to improve brain iron stores.
How Dopamine Signaling Goes Wrong
Dopamine dysfunction sits at the center of RLS, and the pattern is more nuanced than a simple shortage. The substantia nigra and a cluster of nearby brain structures called the striatum rely on dopamine to regulate movement and sensory processing. In RLS, the number of dopamine D2 receptors in the striatum is reduced, and these receptors appear to be iron-dependent, meaning they underperform when brain iron is low.
This creates a feedback loop. When daytime dopamine levels are relatively high, the brain compensates by dialing down the sensitivity of its dopamine receptors. That adjustment works fine during the day. But in the evening, when dopamine naturally drops to about 60% of its morning peak, those already-downregulated receptors can’t keep up. The result is a functional dopamine deficit that’s worst in the late evening and around sleep onset, exactly when RLS symptoms tend to flare.
Why Symptoms Peak at Night
The hallmark timing of RLS, worse in the evening and at rest, isn’t random. It follows the body’s circadian rhythms in multiple overlapping ways. Dopamine levels in the blood peak around 8 a.m. and gradually decline, hitting their lowest point around 3 a.m. Iron levels in the body also dip in the evening. Both of these troughs coincide with the window when RLS symptoms are most intense.
Melatonin, the hormone that rises in the evening to promote sleep, adds another layer. It suppresses dopamine release by blocking calcium flow into nerve endings. So just as your brain’s dopamine is naturally falling, melatonin actively pushes it lower. For someone whose dopamine system is already compromised by low brain iron and reduced receptor function, this evening convergence tips the balance into the uncomfortable sensations and restlessness that define the condition. Core body temperature follows a similar pattern: RLS symptoms peak when body temperature is at its lowest, further confirming the circadian link.
Genetic Risk Factors
RLS runs in families, and large-scale genetic studies have pinpointed specific regions of DNA that increase susceptibility. The first major genome-wide study in 2007 identified three genes strongly associated with the condition: MEIS1, BTBD9, and SKOR1. These findings were replicated independently in populations from Germany, France, Canada, Iceland, and the United States.
MEIS1 is a gene that helps regulate how other genes are expressed during development, and it directly controls the activity of SKOR1, a gene involved in nervous system signaling. BTBD9 has been linked not only to RLS but also to periodic limb movements during sleep, the involuntary leg jerks that often accompany the condition. Having risk variants in these genes doesn’t guarantee you’ll develop RLS, but it increases the likelihood, especially when combined with environmental triggers like iron deficiency.
Secondary Causes and Medical Conditions
Several medical conditions substantially raise the risk of developing RLS. Kidney disease is among the strongest triggers. In patients on dialysis, RLS prevalence ranges from about 13% to as high as 65%, depending on how adequately dialysis is delivered and how well iron deficiency is managed. Inadequate dialysis, measured by a technical marker of treatment dose, is independently associated with worse RLS symptoms.
Pregnancy is another common trigger. Between 10% and 34% of pregnant women develop RLS, likely driven by the increased iron demands of pregnancy and hormonal shifts that affect dopamine. The good news is that symptoms typically resolve within the first four weeks after delivery, though a small number of women continue to experience them postpartum.
Peripheral neuropathy, or damage to the nerves outside the brain and spinal cord, also overlaps significantly with RLS. About 22% of people with peripheral neuropathy report RLS symptoms, and roughly 42% of RLS patients show signs of nerve damage when tested. This is particularly common in people with diabetes, kidney disease, and certain autoimmune conditions. Some research suggests that damage to small sensory nerve fibers may contribute to the uncomfortable leg sensations, though it’s still unclear whether this represents a distinct subtype of RLS or part of the same underlying process.
Medications That Can Trigger or Worsen RLS
A number of commonly prescribed medications are known to provoke or intensify RLS symptoms, and the thread connecting most of them is their effect on dopamine or related brain chemicals.
- Antidepressants: SSRIs and SNRIs are frequent culprits. Fluoxetine, sertraline, paroxetine, citalopram, escitalopram, venlafaxine, and mirtazapine have all been linked to worsening symptoms. Older tricyclic antidepressants carry similar risk.
- Antipsychotics: Medications that block dopamine receptors, such as olanzapine, risperidone, and quetiapine, can trigger or aggravate RLS by directly interfering with the dopamine signaling that’s already compromised.
- Antihistamines: Over-the-counter allergy and sleep medications that act on H1 receptors, the kind found in diphenhydramine (Benadryl), are associated with increased RLS symptoms.
- Anti-nausea drugs: Metoclopramide and prochlorperazine, which block dopamine as part of their anti-nausea effect, can also make RLS worse.
If you’re taking any of these and notice your symptoms worsening, the medication may be a contributing factor worth discussing with whoever prescribed it.
Putting the Causes Together
RLS is rarely caused by a single factor. The most common pattern involves a genetic predisposition that makes the brain’s dopamine system more vulnerable, combined with iron deficiency that impairs dopamine production and receptor function. Circadian rhythms then amplify this dysfunction in the evening, when dopamine and iron levels naturally fall and melatonin rises. Layer on a secondary trigger like pregnancy, kidney disease, nerve damage, or a dopamine-blocking medication, and the threshold for symptoms drops further. This is why two people with similar iron levels can have very different experiences: their genetic background, medication use, and coexisting health conditions all shape whether the dopamine deficit becomes severe enough to produce that unmistakable urge to move.