South Korea is known for their education system, which includes very long days of up to (or over) 12 hours. Because of intense competition and cultural factors, high school students are encouraged to do schooling beyond the typical 8am-4pm schedule.
As high schoolers near their graduation, it's common for them to get only 4 or 5 hours of sleep per night because of their rigorous schedules. After going to school from the normal hours of around 8am-4pm, students typically go home for a quick meal only to leave for night classes at a private school. When the day is over, they return home well after midnight before rising again early in the morning to start the new day.
There is a saying in South Korea that "If you sleep three hours a night, you may get into a top 'SKY university; if you sleep four hours each night, you may get into another university; if you sleep five or more hours each night, especially in your last year of high school, forget about getting into any university."