Brief Answer:
In BPD II patients it may be a co-morbid illness

Detailed Answer:
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Although no definitive studies are there which proves that Restless Leg Syndrome is common in BPD patients but basic biochemical abnormalities in these disorders can support that in patients of BPD restless leg syndrome is common.

In a number of studies Dopamine depletion is thought to be a cause behind Restless legs. Depletion of Dopamine is also a cause of anhedonia (feature of depression) and some symptoms of hyperactivity. In patients of Bipolar disorder it is a common finding and a lot of drugs which improves concentration of Dopamine are useful. So we can say that in both disorders basic biochemical abnormality is same to some extent.

Second view to support this answer is that reduced latency of REM sleep is a common trait marker of depression or in simple words in patients of depression these is reduced latency of REM and increased cycles of REM sleep. Restless leg syndrome is a condition of REM sleep. So both disorders are commonly found to be co-morbid.

Drugs which improves sleep architecture are very useful in both BPD as well as in restless leg syndrome as Ramelteon, melatonin etc.

Hope I cleared your doubt.
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