A large body of evidence documents ill effects on youth who don’t get enough sleep, reports Psychology Today.

It’s nearly time for kids across the country to head back to school and that means—for most kids—waking up bright and early. While early mornings are not often problematic for younger kids, tweens and teens often struggle to get up in the morning.

Science demonstrates that when young people begin puberty, their biological clocks shift; they typically become sleepy later—as late as 11 p.m.—and need to sleep later in the morning to get the recommended 8 to 10 hours of sleep each night.

Yet the vast majority of middle and high schools in the U.S. begin before 8 a.m. If you factor in travel time, it means many young people have to wake up in the early morning hours.