I am currently seeing a Naturopath (ND) for the same issue. So far, I've been tested for allergies, vitamin deficiencies and thyroid issues. All of which I have some of. I am also in the process of collecting saliva samples to test my reproductive hormone levels, cortisol and melatonin. What my treatment plan will end up being, I don't know, but I feel that each little bit of testing is helping. And I am here b/c my regular doctor and insurance wouldn't do the things I needed them to do and simply, didn't understand what I needed or how to do it. I've really had to do a lot of learning on my own and advocating for myself. I suggest two resources that should be a great help to you: The Hormone Diet, by Natasha Turner, ND and the website www.womentowomen.com. Both of these resources talk about the confluences of diet and bodily systems and make recommendations that our regular doctors normally overlook.
So, it could be anything that anyone above mentioned. It could be a combination of things. I just encourage you to think outside the box in terms of seeking help.
Also, as a psychologist, I understand the importance of knowing WHY you aren't sleeping and not just taking some pill to fix it. You start taking some sleeping pill and then you need more and more. If you understand why, you can at least address the root of the problem.
Lastly, I've tried Ashwaganda Root with some success. I took it at night with my calcium/mag supplements. That has really helped me get to sleep a lot better. I still have trouble staying asleep, though. But, I'm working on it!
Good luck to you!