I don't know what is wrong, but I can give you my deepest sympathy. I'm going through the same thing. It can be just awful.

In my case, it seems to be related to getting older. I've heard about this problem from lots of other people over age 50. For someone younger, I would consider that the quality of the night can have a lot to do with the quality of the day that preceded the night. Two very important things are to get enough activity and enough meaningful interaction with other people.

Do you have anxiety issues?