I'm sure there's numerous potential reasons for it. Some may be relying on the consistency of their sleep schedule more than others, have responsibilities that reduce how much they can reasonably fit in their schedule, may not care about the quantity or quality of sleep they get, have a pre-existing sleep debt, have health issues that compound with sleep changes, alternative sleep schedules, genetic predispositions, or literally anything else related to how sleep, physical health, mental health, and bodily systems function.

If you think of how one thing being shifted can set other things way more off balance then it makes sense. I'm not an expert in any of this but it's definitely a complicated topic at the very least.