As you point out that's a multi day commitment, and if part of the volunteers either adjust the timing of the experiments to match specific weeks (e.g. parents choosing school vacations), or adjust their schedule accordingly, what is measured becomes fundamentally different in nature to what measuring longer periods would bring.
I'm with you on how we don't have a choice regarding to the quality of the study, it's just crazy hard to get any data at scale. But we can look at it as a very flawed "best of what we can do" and not take the patterns too seriously.
Is 500k brits for 1 week as good as 5k brits for 100 weeks.
Effectively with so much data aren't you getting a superposition anyway.
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.
The weeks that were measured in this study were not random. How far from random is the big question.
In the similar way sampling the height of 1000 of 1M people will give you a good estimate of the average.