On joining one of my classes in April a new student, who had never done any yoga before, told me almost straight away that since she had started coming to yoga she had stopped waking up at 4am in the morning.
Another student, in a different class, with the same problem told me that after coming to yoga she would sleep right through the night for the next three or four nights. I suggested she try doing just the warmups that we do at the start of the class halfway through the week and see if this was enough to give her undisturbed sleep for the whole of the week; it was.
Disturbed sleep seems quite a common occurrence and I'm certainly not suggesting that it would work for everyone, but it does seem worth a try.
Do I know what causes the problem in the first place? No. Do I know why yoga helps? No. But the the same can most likely be said of medical science.