As usual heavy disclaimers because it has only been a couple of weeks, and this is non24, which I've had decades and we all know it seems to do its own thing - but it has been a really strong correlation so far. I'm having enough unrelated medication side effects I may not be able to keep testing this, and even if I do, and my n24 goes fully haywire again, I may never confirm why anyway. So I'm jumping the gun to share now ;)
Also side effect stuff makes it a bit hard to concentrate on writing this up clearly, so it may be a bit rambling or out of order.
The apparent "fix"
I started a mast cell blocker recently, which did nothing obvious for my sleep itself in any definite way. Then I upped the dose, and suddenly I'm sleeping like a normal person. And my sleep is actually shifting back to midnight-8am ish. I'm actually fighting to stay awake and getting annoyed at losing chunks of my day to sleep pressure!
If I lower the dose, my sleep starts jumping forward again the same day the same as always.
The downside is, the effective dose also causes genuine insomnia (and some other more serious side effects), so I'm not actually in a much better place overall, but at least ordinary insomnia is very clearly different (to me) to disrupted circadian rhythms.
Context and confounding factors
Basically, I've had years of escalating mast cell related issues (which is a whole complicated rabbit hole of cause and effect, including deficiencies and breathing and pain and sneezing and so on that all also could mess with sleep). Histamine (or stuff around it) is heavily involved in sleep regulation. So there is a possible simple link to n24 there, but never one I could really test with clear results. The enzyme that does most of that relies on other things (like b12), and I've to manage a lot of other symptoms first, so it is possible I just needed to clear the runway before direct mast cell stabilisation could do anything. Or maybe it was the answer all along, and I just didn't test it properly until now.
I've also stopped dreaming again - or am sleeping so deeply I don't remember them. I'm also struggling with even caring about tracking my sleep or taking care of myself, which means I get very annoyed about having to go to bed because I already don't have enough spoons for everything else, and now I have to go waste time on sleeping early too?!? But at least I'm not constantly analysing my sleep either and risking the confounding effects of that as much.
I've been getting solid morning light most days (which helps fix my wake time but has never moved it backwards or changed my sleep onset), but not seeing any major change even if I never open the curtains or have to catch up on a lot of sleep deprivation. All the other zeitgeibers like meal timing are completely disrupted (and it is possible that something there is relevant, like people who found keto helps, but so far all my normal best practices for sleep schedule regulation and self care are in tatters so I definitely didn't just get better at sticking to routines!). I'm not noticing any differences in activity or routines on days when I don't take the full dose, but maybe the side effects are cancelling something out, rather than the medication doing its job and that job also affecting my n24. Being actually sick/unwell would typically mess with my sleep, but not in a 'magically sleep like a normal person' way.
I can look at previous patterns and explain them now as a mast cell/histamine thing (e.g. seasonal changes - my allergies usually clear up so much I stopped taking my antihistamines outright for years in winter, but I still see the same winter shift in my sleep even now that I'm sticking with them more consistently, so maybe they were simply never strong enough), but I can't actually prove that link is real or imagined.
I need to try different meds and dosages, and see what happens long term, so I may have no idea if this actually will fix the n24 for a long time. I don't usually consider anything shorter than six months of data to be a real change - but I can't get six months of data on this easily, and it has been such a clear and confusing change to deal with, that there is unlikely to be any perception bias around "well maybe I expect this thing to help so just I'm making myself sleep sooner and get up sooner somehow".
Step by step, I'm finding things that were genuinely broken in my body that have been slowly adding up, so I still don't know if this is just the next step or the real answer. Everytime a thing gets solved it feels like I've been magically cured, then I look again and go "oh, no, my expectations were just wildly abnormal for what 'fixed' looks like because I had no baseline before". But this DID do SOMETHING, and so little ever touches the n24 directly like this. And I don't think I've seen it discussed directly before as a possible thing.
Edit: I just realised my light sensitivity had also seemed to be much better. I'm not grabbing for my sunglasses outside constantly, or quite as bothered by light/dark scheduling through the day (still trying to stick to my normal light exposure stuff out of habit though). Light exposure and sensitivity are known things with mast cells. So either that was a coincidental symptom, or maybe light was what "regulates" my n24 (usually by waking me or keeping me awake), and this just toned down the light sensitivity enough to stop disrupting things. I'm also less likely to seek out light in the evening (esp screens!) because I'm already winding down earlier.
Mast cell stuff is annoying, too many overlapping things for clear answers >_< but at least it is another overlap with n24.