It's supposed to be related to dopamine, which tends to be lower at the end of the day and can drop quite suddenly for a variety of reasons. It also has a number of triggers.
I had it very seriously, every night for months and it affected my sleep, which is bad anyway. The identified causes were diabetes, iron deficiency, high blood pressure medication, caffeine and stress.
I was prescribed ropinirole, which helped, iron supplements, had my blood pressure medication changed, I drink less coffee and I do yoga in the evening. The ropinirole was reduced, so I don't take it at all now and, thank goodness, I haven't had RLS for about six months.