I wonder how much of this is driven by confounding variables they haven't accounted for.

They do factor in shift work as a categorical variable, and employment status as a categorical variable not taking into account occupation. But probably occupation (not a variable here) interacts with sleep status. Any job that involves a lot of flying (pilot, crew, people travelling for business) get more cosmic radiation exposure, for example, and potentially more sleep disruption. Certain operations and manufacturing jobs correlated with exposure to carcinogens also likely correlate with less regular sleep, possibly in a way that isn't corrected for by the limited shift work categories.

Yeah, this type of observational data often falls over when you dig into the details. And yes, they say they adjusted for a number of variables, but even if hypothetically they perfectly adjusted for all possible confounding variables (highly unlikely IMO), it's possible that the causation simply runs in the other direction: poorer quality sleep and variable sleep cycles could just be a symptom of illness or being generally unhealthy, not the root underlying cause.

If you did read the paper, you'd have noticed that people who slept more regularly in this research were also wealthier, and more physically active. They did say they adjusted for these variables, but who knows. Maybe they didn't adjust them enough, maybe there are more covariates they didn't consider.

usually these studies are done across a very narrow sample of Earth's population. often from similar region or country etc. Which dont get me wrong, is a lot of stuff to process for such a study, but the number is statistically insignificant. many more similar studies would need to show identical results.

this one selected about 100k people from a dataset of around 500k. All from one country/region (UK)

furthermore they dont measure sleep but they estimate if someone was maybe asleep based data from an accelerometer. so they cannot measure what sleep state someone acheived or if they were actually really sleep or just u know staring at the ceiling in an existential crisis....

The confounding variable I think is most important is that many people have an internal clock that is shifted or lengthened relative to other people in the same house.

If others in the house prefer to sleep from 10-6 and you prefer to sleep from 12-6, but others start making noise a 6, your sleep quality in the last two hours is destroyed. Then over time, it just results in poor sleep regularity, as you cycle between exhaustion and trying to sleep according to your internal clock.

I thought the same. If your life is so in order that you routinely sleep on the same interval, perhaps your life is not as stressful as others who sleep more chaotically

they had controls