Earlier quoted context omitted.
You're forgetting transportation to and from school in your daily calculation, which could take up to an hour and a half total. And socialization which is non-negotiable. At least an hour for that? Oh and some people might need 9 hours of sleep, and another HN comment actually cited that this is recommended for a 13 year old. You see the problem with cutting it so close that homework just, just barely fits in the rem…
I did an experiment in college where I tried to see just how much work I could take on and stay sane. I'm very successfully academically (PhD at 26 from a very respected institution), and I found that if I did not have at least eight hours of unscheduled, unplanned time to just do nothing useful it severely impacted my ability to perform on any task. And I'm an extreme outlier in terms of innate skill, I say without…
Additionally, innate skill does not translate to the ability to handle stress, discipline, or work ethic. Your schedule is entirely unrealistic outside of academia today (or being your own boss), judging from the population of successful people (in finance, software engineers, researchers, politicians, etc) you'll see that most people get by with less than 2 hours of unscheduled time whether forced or by choice.
Personally, I find unscheduled time and being mindless hampers my productivity. I function best under pressure and produce my best work. The most academically successful year of my life was when I was spending 80 hours a week on research, class, and studying (1-2 hours). My point in this is that what works for you is not necessarily the minimum requirement for success.
Also, I am aware of the physiological consequences of stress before anyone gets to that point.