While the federal recommendation is making headlines, the data on the potential risks of chronically tired adolescents isn’t new information. Indeed, the research has been accumulating steadily for years, including some recent large-scale studies.

As the Minneapolis Star-Tribune reported in April, the University of Minnesota’s Center for Applied Research and Educational Improvement “finally put to rest the long-standing question of whether later start times correlate to increased academic performance for high-school students”:

Researchers analyzed data from more than 9,000 students at eight high schools in Minnesota, Colorado, and Wyoming and found that shifting the school day later in the morning resulted in a boost in attendance, test scores, and grades in math, English, science, and social studies. Schools also saw a decrease in tardiness, substance abuse, and symptoms of depression. Some even had a dramatic drop in teen car crashes.

Here’s what the research shows: Adolescents’ “internal clocks”—the circadian rhythms that control a human’s responses to stimuli and determine sleep patterns—operate differently than those of other age groups. It’s typically more difficult for adolescents to fall asleep earlier in the evening than it is for other age demographics. And while teenagers are going to bed later, their school start times are often becoming earlier as they advance through middle and high school.

In a landmark study in 1998 of adolescent sleeping habits, the Brown University researcher Mary Carskadon followed 10th-graders who were making the switch to a 7:20 a.m. start time, about an hour earlier than their schedule as ninth-graders. Despite the new schedule, the students went to bed at about the same time as they did the year before: 10:40 p.m. on average.

Carskadon’s team found that students showed up for morning classes seriously sleep-deprived and that the 7:20 a.m. start time required them to be awake during hours that ran contrary to their internal clocks. Fewer than half of the 10th-graders averaged even seven hours of sleep each night, which is already below the recommended amount. Indeed, Carskadon’s team concluded the students bordered on “pathologically sleepy.”

So, if the science is so strong, what’s getting in the way of changing the policy?

Carskadon, a professor of psychiatry and human behavior, notes that passionate arguments abound on both sides of the debate—just about all of which she’s heard over the years. In some districts, the start times are largely dictated by local transportation companies, with school boards and superintendents contending they lack the funds or authority to change things. Meanwhile, parents are often reluctant to have teens start later, whether because they rely on having older children at home in the afternoons to take care of younger siblings or because they’re concerned that it will interfere with extracurricular opportunities. Indeed, there’s always a vocal chorus warning that later start times will hurt high-school sports.