> 2) while I don't have an easy reference, a number of studies have found that even strenuous exercise before bed does not seem to impair sleep. As with everything sleep related there could easily be lots of individual variation but in general I don't think it usually causes trouble.

Hopefully.

These days, with all this working and staying at home, I'm sure for a lot of people their most strenuous exercise is just before going to sleep.

> In my experience, when you wake up seems to be more important for setting the circadian rhythm than when you get to sleep.

That's my experience too, however I think it is misleading, for some people, to describe circadian rhythm as a single daily clock.

In my experience it's more like:

1) There are a small number of distinct rhythms. When these are aligned, sleep is more refreshing, and they feel like one larger coherent process. When they are unaligned, sleep is less refreshing, and the awake time has a number of "different kinds of tired" intervals, as varies cycles that would ideally be during sleep occur during awake time instead. When severe, it's difficult or impossible to work during these intervals, even when getting plenty of sleep as well.

"Different kinds of tired" means things like tired muscles, or tired mind, or difficulty focusing, or poorer temperature regulation, or... Basically different processes in the body, which could all be conflated as "tired" yet are distinct from each other.

One of the most annoying is feeling very sleepy and needing to lie down, yet not falling asleep because it's the wrong kind of tired for actual sleep. After hours of that, maybe the right kind of tired will happen, at which point actual sleep happens. When these cycles are misaligned the overall process can take most or even all of the available 24 hours a day; it is very difficult to hold down a job when this is a regular occurrence.

2) The overall circadian cycle sometimes breaks up into pieces, like those 24 hour mechanical timers with a separate bit of plastic for each 15 minutes, which can be set to one long interval or more than one shorter interval.

As many other people point out, regularity is good for maintaining a strong circadian rhythm. I find that includes these multiple intervals: They seem to appreciate consistency too. I also find waking earlier/later (e.g. due to the sun, work, clocks etc) doesn't always drag the whole 24 hour cycle forward or backward; sometimes it seems to create a new interval and allow an old one to shrink or fade.

> trigger event and that once triggered is essentially irreversible

I had a trigger event which I can trace back specifically to age 15: Working on school coursework in bed in the early hours of the night. It's not really a single event so much as a time period of a few months.

Up until that point I had reasonably regular sleep.

Ever since then I suffered from essentially chaotic and unpredictable sleep times for about 20 years.
No amount of "discipline", from school, jobs etc., fixed this. It just meant I was tired, and late getting to work a lot, because the alternative is to be on time but too exhausted to be useful, a real dilemma.

Eventually after about age 35 it started getting a bit more regular. But it's not great, and I have to be very conscious about managing sleep.

A challenging aspect of this is I'm far more productive if I decide to pull an all-nighter than if I remain disciplined about sleep. I have found with great consistency that in a well-chosen all-nighter, I've often got more things sorted out than a month of regular work. It's frustrating; I would like to do that in the daytime! If I'm disciplined and avoid doing that, months can go by with only "mundane" levels of productivity, and it's disappointing knowing I'm capable of more. As a decent programmer, I'm lucky that "mundane" is still good, but it's not what I can do at my best. I'd still like to crack this problem but I haven't found a solution yet.