According to traditional Chinese medicine, the cause of restless leg syndrome is heat in the Heart causing your spirit to become agitated. The end result is that you experience restlessness when trying to sleep.
The following "statements of fact" supports this idea. All of the big words on this page come from one of a few classic texts in Chinese medicine that date back to the time of Christ. Perhaps that's why some of us call the "Huang Di Nei Jing" the "bible" of traditional Chinese medicine.
In TCM, certain organs store stuff. In biomedicine things like nutrients are stored in fat cells. In Chinese medicine the Shen is stored in the Heart. The Shen is the Spirit. That can suggest something holy and eternal, but clinically speaking it also applies to your personality, demeanor, memories and certain cognitive functions. This spirit is stored in the Heart. In biomedicine, they'd put most of these "spirit" functions into the brain and/or central nervous system.
In TCM, each organ is hypersensitive to one unique type of weather. In the case of the Heart, it loves to feel warm, but hates to feel hot. When it feels hot, the spirit that it stores can become restless. "Five element theory" describes the different environmental factors that effect the organs in the body according to this theoretical tradition.
According to the theory of Zang-Fu (which essentially translates to "internal organs") the Kidneys are the source of yin and yang in the body. Yin is body fluids and cooling mechanisms in this context. Yang is metabolic heat for this discussion. When Kidney yin is deficient, the body can get hot giving rise to hot flashes and night sweats. This is something like when your car's radiator is low on fluid, the car can overheat. This doesn't happen because the engine is running too hot, but because the radiator yin, or fluid is deficient.
Because the Kidneys are the source of Yin, when the Kidneys get dry, the Heart gets dry also. When the Heart gets dry, the spirit that is stored in the Heart has nothing to anchor it down as the Heart yin generally does. I can't quite think of a good analogy for this. Needless to say, the spirit as a substance is light and airy. One of the jobs of the yin of the Heart is to give the spirit a nice moist place to reside. When that moisture is deficient, the spirit becomes flighty, fidgety, and a little bit over dramatic. Other symptoms of Heart yin deficiency include panic attacks, heart palpitations, anxiety, and insomnia.
Back to the radiator fluid causing the car to overheat. When the body gets hot because of its lack of cooling mechanisms, that heat will rise in the body to the Heart where it agitates the spirit and causes restlessness. Between this mechanism and the Heart yin unable to anchor the spirit, it can become a real problem emotionally and physically.
When the Kidney yin is deficient all the other organs that rely on body fluids for their balanced function become dry. Most often, it is the Liver that suffers first from the Kidney yin deficiency.
Just like the Heart can get too hot when there aren't enough fluids to cool it, the Liver too can generate some problems when it gets dry. The problem it causes is called internal wind. Internal wind makes our hands, feet, or other structures shake and tremble. Biomedicine would described these things mostly in neurological terms (nocturnal myoclonus). Internal wind can look like anything from a tic in your eyelid, to periodic limb movement disorder (which often arises with restless legs), to some forms of epileptic seizures.
Please do not assume that if you have PLMD that you need to be treated for epilepsy. This is wrong even though there are some similarities according to TCM theory. Periodic limb movement disorder (PLMD) is characterized by involuntary leg twitching or jerking movements during sleep that typically occur every 10 to 60 seconds, sometimes throughout the night. This is the official descriptino of PLMD, however some patients report also that these twitching muscles are not limited to the limbs, but can arise in the abdomen among other locations.
So why does Kidney yin become deficient? There are a few causes. If you recently had a high fever and your urine is now looking darker than usual, you're dehydrated. Drink more fluids, take some herbs if you want for your yin, you'll get over it. However there is another cause for a Kidney yin deficiency. This cause is called Kidney jing deficiency. Jing is sometimes translated as "essence". You may recall that the Heart stores the shen. Well, the Kidneys store the jing. Jing has a number of functions. One of which is to stimulate periods of growth in our lives. For instance, when your reproductive organs start to mature this is moderated by the Kidney jing. Menopause is timed by the jing too. This jing is also the precursor to Kidney yin. So, a jing deficiency can cause a yin deficiency.
This is why restless leg syndrome has a tendency to effect the elderly more than the young. It is because their jing is naturally being used up, so they develop a deficiency of yin causing all of the issues that have been described above.
Kidney jing comes mostly from our parents, and it is passed on to our children. If our parents have a tendency toward jing deficiency, it is likely that we will too. Because as we get older we have less jing, the issue of children with a jing deficiency increases with age. According to TCM, this is why some birth defects favor the children of parents who are older such as Down's syndrome.
A family history of RLS is seen in approximately 50 percent of such cases, suggesting a genetic form of the disorder. People with familial RLS tend to be younger when symptoms start and have a slower progression of the condition. This supports the idea that RLS comes due to a jing deficiency. If you get it from your parents, it will arise earlier in life. If you are just naturally running out of jing (as we all do) it'll show up later in life.