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 exposu…
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.
In the past few years I went from having relaxed 11AM daily meetings to rigid 8AM meetings, and my sleep has suffered immensely. But nothing else in my life has changed, so it wouldn't show up in my socioeconomic data.