Hi Jandy23
I had exactly the same problem but it didnt start until 4 months are taking tamoxifen, I think mine was the menopause symptons rather than the tamoxifen.
I found that staying up later then sleeping in helped me to get enough sleep ,as i was becoming forgetful and had a not all there feeling and couldnt concentrate, and that was stressing me out ,its a bit of a vicious cycle .And once i felt in control of my sleep again i no longer have the problem.
Hopefully its just temporary for you as well.
best wishes x