Armed with new research showing adolescents who don't get enough sleep are more at risk of physical and mental health problems, more automobile accidents and poor academic performance, a group of pediatricians in the United States is reviving the call for later school hours.

In a new policy statement published online Aug. 25, the American Academy of Pediatrics is recommending middle and high schools start classes 8:30 a.m. or later, according to a news release.

Such changes, the medical organization argues, would better align school schedules to the biological sleep rhythms of adolescents, whose sleep-wake cycles begin to shift up to two hours later at the start of puberty.

"Chronic sleep loss in children and adolescents is one of the most common, and easily fixable, public health issues in the U.S. today," pediatrician Judith Owens, the lead author of the policy statement, said in the release.

The opinion piece is published in the September 2014 issue of the journal Pediatrics.

"The research is clear that adolescents who get enough sleep have a reduced risk of being overweight or suffering depression, are less likely to be involved in automobile accidents, and have better grades, higher standardized test scores and an overall better quality of life," Owens said. "Studies have shown that delaying early school start times is one key factor that can help adolescents get the sleep they need to grow and learn."

Several studies have shown the average U.S. adolescent is chronically sleep-deprived and pathologically sleepy. In fact, a National Sleep Foundation poll found 59 percent of 6th through 8th graders and 87 percent of high school students in the country were getting less than the recommended 8.5 to 9.5 hours of sleep on school nights.

The reasons why teens tend to lack sleep are complex, the statement asserts, and include homework, extracurricular activities, after-school jobs and use of technology that can keep youths up late on week nights.

The evidence strongly suggests that starting the school day too early is a significant contributor to chronic sleep deprivation, with an estimated 40 percent of high schools in the U.S. currently starting instruction before 8 a.m.

The AAP urges middle and high schools to aim for start times that allow students to receive 8.5 to 9.5 hours of sleep a night -- so in most cases, that would imply a school start time of 8:30 a.m. or later.

"The AAP is making a definitive and powerful statement about the importance of sleep to the health, safety, performance and well-being of our nation's youth," Dr. Owens said. "By advocating for later school start times for middle and high school students, the AAP is both promoting the compelling scientific evidence that supports school start time delay as an important public health measure, and providing support and encouragement to those school districts around the country contemplating that change."