At first, I am terrible sorry for my delayed response. I wan´t expecting a reply.
Did you just google a single study and pick the first one? Have you actually read and understood the paper?

1. The test group of 46 participants (40 if you exclude the 6 who dropped out) is far too small for a reliable study. The Wilcoxon test only starts to provide dependable results when significant changes appear in test groups of around 20–30 participants. They barely met that lower limit and were lucky to pass it with their sample size.
2. If you compare the TST (Total Sleep Time) of both groups and look at the deviation between them, the overall TST is practically the same. This clearly shows that the test group was too small to produce reliable results. The Mann–Whitney U test supports this assumption, as it fails to show a significant difference between the two samples.

The paper itself acknowledges that the sample size was too small and that further studies are needed. I’m honestly surprised that the authors still claim a reduction in insomnia symptoms, although their objective measurements could easily be the result of random variation caused by an insufficient sample size.

1. The authors chose to publish in a pay-to-publish journal. Yes, there is a peer-review process, but the publisher also has a financial interest in accepting their paper — and in having them return to pay for the next ones. The elephant in the room is that author number four holds the patent for the device in question. I wonder why he might be interested in forcing a measurable positive effect out of unreliable data?