I don't doubt that the findings are the case, I've experienced it myself and seen it in plenty others. What I would've liked to see in the paper, though, is more proof of the cause-and-effect relationship's weight here: do people who naturally stay up late get worse grades, or do people who get worse grades end up with worse sleep habits?

I'd guess there's some degree of both, but without the study being more longitudinal (eg, tracking the same population across their entire time in college to see how their grades and sleep habits relate) it's hard to say. Purely anecdotally, I've seen people who start struggling (for reasons that have nothing to do with sleep) drift into a depressive/withdrawn existence that includes insomnia and sleeping late.