Parents, students and teachers often argue, with little evidence, about whether U.S. high schools begin too early in the morning. In the past three years, however, scientific studies have piled up, and they all lead to the same conclusion: a later start time improves learning. And the later the start, the better.

Biological research shows that circadian rhythms shift during the teen years, pushing boys and girls to stay up later at night and sleep later into the morning. The phase shift, driven by a change in melatonin in the brain, begins around age 13, gets stronger by ages 15 and 16, and peaks at ages 17, 18 or 19.

Does that affect learning? It does, according to Kyla Wahlstrom, director of the Center for Applied Research and Educational Improvement at the University of Minnesota. She published a large study in February that tracked more than 9,000 students in eight public high schools in Minnesota, Colorado and Wyoming. After one semester, when school began at 8:35 a.m. or later, grades earned in math, English, science and social studies typically rose a quarter step—for example, up halfway from B to B+.

Two journal articles that Wahlstrom has reviewed but have not yet been published reach similar conclusions. So did a controlled experiment completed by the U.S. Air Force Academy, which required different sets of cadets to begin at different times during their freshman year. A 2012 study of North Carolina school districts that varied school times because of transportation problems showed that later start times correlated with higher scores in math and reading. Still other studies indicate that delaying start times raises attendance, lowers depression rates and reduces car crashes among teens, all because they are getting more of the extra sleep they need.

And the later the delay, the greater the payoff. In various studies, school districts that shifted from 7:30 to 8:00 a.m. saw more benefits than those that shifted from 7:15 to 7:45 a.m. Studies in Brazil, Italy and Israel showed similar improvements in grades. The key is allowing teens to get at least eight hours of sleep, preferably nine. In Europe, it is rare for high school to start before 9:00 a.m.

Studies also show that common arguments against later start times ring hollow. In hundreds of districts that have made the change, students do not have a harder time fitting in after-school activities such as sports or in keeping part-time jobs. “Once these school districts change, they don't want to go back,” Wahlstrom says.

Even “the bus issue” can work out for everyone. Many districts bus kids to high school first, then rerun the routes for the elementary schools. Flipping the order would bring high schoolers to class later and benefit their little sisters and brothers; other studies show that young children are more awake and more ready to learn earlier in the morning.