I mean the data lacks any person's sleep history, but if there is a cohort of bad sleepers, as a group you get a collective pseudo-history (because altough study is done at the same time they are in different life stages) that is no-one's history but might track the average well for that cohort AND take into consideration more than a week.
In the similar way sampling the height of 1000 of 1M people will give you a good estimate of the average.
In the similar way sampling the height of 1000 of 1M people will give you a good estimate of the average.