Considering the importance of the subject, it still seems important to bring this out, especially if there's conclusions that if you have irregular sleep you are significantly increasing your odds of dying earlier. I think intuitively we can know how important sleep is and I desperately want to sleep better. It's fine to speak of it being predictor etc, and all that, then they end the abstract with "Sleep regularity may be a simple, effective target for improving general health and survival.", which is technically fine since they use "may", although later articles based on the study will likely make the statement more confident.

But I guess what might be slightly triggering is claiming that it's a "simple" target. Don't I wish I could sleep on command and better?

So a clear question is - why do people choose to sleep or why do they naturally sleep irregularly?

Because for that there must be a logical cause in the first place. They say they control for mental health and all that, but is it then that ultimately it comes down to preference in their mind? I'd think most people want to sleep in healthy way.

Basically - if they were able to control for all possible confounding variables, what exactly was the cause of irregular sleep?

Anecdotally I can say that I sleep more irregularly the more stress there is, and stress could easily affect health, but if they controlled for stress, what then?

I guess ultimately they are saying it's a desirable target to measure, so it's fine in that sense. They are not really saying that choosing to sleep irregularly is what is causing the issues.

But you can just apply this to anything. I feel unlike unless you're an insider with skin in the game, your criticism doesn't land other than a generic surface one.

To be clear, I apply an equal deep skepticism to most fields that aren't math (in the sense of a priority) or physics (in the sense that you aren't trying to study the entire world, but a specific set of phenomena that you can reliably control enough + repeat to run intervention on), whether the results agree with me or not. Maybe a bit of intellectual closed-mindedness. But then that means that me, personally, I can't in good faith use the criticism as a proper 'debunk' argument - at best it's a heuristic to avoid spending cycles to evaluate it (which is 'rational' behavior, as much as I hate that word, IMO).

McElreath's Statistical Rethinking is also a great reference for causal thinking and modeling.

Do you even bother to click on links before asking a question?

There is a whole paragraph on "Statistical analysis" that even provides five supplementary methods (S1.5, S2.2 and S2.4-6) if you want detailed information.