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.
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....
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.