There is a lot of research that lack of sleep (under 7 hours, likely 7-9) causes significant problems with memory, working memory, and is directly linked to cancer and Alzheimer's in later years.

In short, it might be feasible in the short term, especially if they have mindless jobs where they can study. I memorized a LOT of vocab during my factory work summers. But it has to be repetitive and mindless for that. I did more thinking at the factory than I did at software jobs.

Anyway, any time anyone says they work 12 hours and study, I don't believe them by default. Once you factor in commuting and maintenance, it's just not realistic...unless you are in a sweatshop dorm, I guess that would save on commuting/grooming/meal prep.