It is not productive to study for 16h/day, or even 10h/day for an extended period. It is not only less effective for sustained learning or intellectual work than spending less but more focused time, it also leads to physical and mental health problems, and sometimes results in severe burnout.
The problem is that (a) students are young and many of them are quite inexperienced with managing their own time and work, (b) students are so stressed and sleep deprived that it is hard to introspect about process or get into a productive rhythm of focused productive work alternated with rest, and (c) there is often a workaholic student culture which creates peer pressure and presents the illusion that staring at a textbook for hours while already half asleep is the mark of a good student. For grad students (especially foreign students) sometimes there is additional pressure from abusive advisors.
It’s sad that e.g. MIT’s unofficial motto is “I hate this fucking place”.
Unfortunately the same kind of culture extends into some people’s professional lives. I dated a lawyer for a while who was a few years out of law school and working for a big firm, and with all the hours she needed to “work” and the few hours she could sleep each night her ability to think through complicated legal arguments or write briefs was severely compromised; sometimes she would be “reading” for an hour before bed with her eyes half closed, barely able to parse the words on the page. But that’s what the firm expected (and by their standards she was performing well), so she felt she had no choice.