Correct me if I'm wrong, but the study itself didn't look near this rigorous. It looks like they made 3 buckets and tried to correlate them with performance. Ho

Correct me if I'm wrong, but the study itself didn't look near this rigorous. It looks like they made 3 buckets and tried to correlate them with performance. How can you do that and they say "aha! look! circadian rhythms"? How do we know that the students who stayed up late aren't also the students that procrastinate and do all their work last minute, and thus recieve lower grades? That seems like just as reasonable a conclusion to draw.