How can Korean students study so much and sleep so little?

I have seen numerous documentaries made by Europeans about study culture in Korea. If they are portraying things correctly, students (also minors) study all day and only sleep for maybe 5 hours per day.

I can’t understand how this is physically possible. You can survive and function without free time, but how can you be productive when you don’t sleep enough, especially in your teenage years? If I sleep only 5 hours per night for several nights in a row, my concentration gets so bad. Am I just a spoiled European with a less strict work ethic?