In statistical mechanics there's a concept of "ensemble average" and its provable that if you have a system, the average state of the system over say a 100 realisations ("ensemble average") run for 1 second each, is equal to the the average of one system run for a 100 seconds - under some assumptions of course.
I don't know enough a about human biology to make a statement about whether any of those assumptions will hold true, but maybe someone else will.
I would also say it's geospecific. Like if there is a nationwide trauma of sorts (politics, etc) it could influence a group to not be representative of the population at large.
That's not the scenario here. An ensemble average requires those 1 second periods to be randomly sampled from the overall runtime. You can't just measure the first second after starting for all samples, or even the 7th second after starting, and say anything statistically sound about the average of the whole runtime.
The weeks that were measured in this study were not random. How far from random is the big question.
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 statistical mechanics there's a concept of "ensemble average" and its provable that if you have a system, the average state of the system over say a 100 realisations ("ensemble average") run for 1 second each, is equal to the the average of one system run for a 100 seconds - under some assumptions of course.
I don't know enough a about human biology to make a statement about whether any of those assumptions will hold true, but maybe someone else will.