From the article: “SRI captures day-to-day variability in bedtime, wake-up time, sleep duration, and awakenings during sleep.”
This has nothing to do with when you sleep, it’s about variability.
>The term "shift work" includes both long-term night shifts and work schedules in which employees change or rotate shifts.
To further show how my comment might be wrong: "In medicine and epidemiology, shift work is considered a risk factor for some health problems in some individuals, as disruption to circadian rhythms..."
I interpreted "shift work" to mean "night shift".
Although you’re probably right in the context of this study, I highly suspect when you sleep does affect things due to the evolutionary and biological significance of the circadian rhythm.