Same here; I've had late-onset insomnia my whole life, and nothing has ever worked to budge it. Ever since my childhood, I've just got used to going to school or work after four or less hours sleep, even none at all - it's just my "normal". It doesn't matter how sleep-deprived I get after days and days of it, I still get to sleep no earlier. Sleeping pills sometimes help a bit, but never for more than a few nights, and they turn me into a zombie the day after. None of the normal recommendations for sleep-hygiene, diet etc. have ever made the slightest difference - in fact, after nearly five decades of it, I find it hard to resist punching people who offer the same tired old advice all the time (of course, they are always people who sleep like logs themselves; I've never known a real insomniac dare to be so impertinent!)
On the other hand, when I don't have any commitments like a job to go to, I settle into a sleep pattern of around 4am to 12noon which gets me a decent 8 hours sleep and is perfectly stable for as long as I'm allowed to stick to it. I never know how much of this to put down to a natural body-clock which is out of sync; having a brain that won't STFU; or just revelling in the peace and quiet of the night-time. It's probably a combination of all three, but the fact that 4am-12noon can be so stable for long periods regardless of my stress levels, or the different environments that I've lived in, makes me think that there probably is something innately off with my brain's sense of timing.
Don't let the fact that I've not found anything put you off from trying different things, though. Sleep disorders are very poorly understood and can have a multitude of causes. One thing you haven't mentioned that may be worth trying is to split your sleep into several shorter periods; it doesn't work for me, but it is very effective for some people and used to be considered perfectly normal in past times.
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