It sounds like your body is going through changes, just as you said. You are doing everything right. You took my complete answer and wrote it yourself.
I'd have said music, ear plugs/ear buds, relaxation exercises, etc....
At this point I'd make an appointment with the general doc. Not the OB/GYN. I only say this because the Melatonin isn't working.
Ask for a mild sleep aid for short term use, maybe 5-10 at most. Then take one every night and get some rest. If you get some good straight hours of sleep with it then your body will do it's job and you'll feel better.
Once you get several days of sleep you can stop taking it and see if your biorhythm has changed back.
I'll say it in writing!!! NOOOO...as we age our bodies simply don't need as much sleep. That's why we age. When we're younger our bodies are constantly changing on a cellular level, dividing cells, multiplying cells, growing and changing. Once we hit that place where our bodies don't do that so much we don't have to sleep so many hours for our cells to heal and regenerate.
We just don't need as many hours and our bodies tend to start staying awake, getting you up before you want, and more.
I always get out of bed when I'm awake. I don't do the computer on the bed, one thing that's a fire hazard, but I want my brain to know bed is for sleep. I don't read in bed for the most part. If I want to go lay down and read or watch TV and it's not bedtime then I might go lay down on the bed all propped up to do that. BUT if it's bedtime I go to bed to lay down and relax.