"From middle school on up, we start to see a correlation between homework and achievement," says Pope. But there's a limit. The correlation fades after 90 minutes for middle school and two hours for high school. "Our research showed that after the 3½-hour mark, you start to see negative ramifications and health issues," says Pope, much of it from sleep deprivation. This, combined with anxiety and stress, can also lead to depression, according to Pope's research. Why is the work taking so long? "Homework should reinforce and extend topics covered in class," says Eskelsen García. "Ask teachers, and they will tell you that too much classroom time is being spent on testing, not enough on learning." Homework is often used to catch up. What's more, overscheduled kids are starting homework late (and exhausted), so it takes longer to do.