Pediatricians have been clear: Early bell times can spell sleep deprivation for teens and, in turn, a decline in academic performance, an increased risk of car accidents and physical and mental health issues.

But according to a recent report by the National Center for Education Statistics, only a fraction of high schools are starting later than 8:30 a.m., which is what the American Academy of Pediatrics recommends.

The average start time for high schools in the United States is 7:59 a.m., according to the report, published Tuesday. For middle and elementary schools, it’s a little later: 8:04 a.m. and 8:17 a.m.

The data came from a survey conducted in the 2015-2016 school year of a nationally representative sample of 8,300 school principals. Only about 13 percent of high schools start later than 8:30. Nearly half — 46 percent — start before 8 a.m.

The report shows that the vast majority of teens are behind their desks before many grown-ups clock in for the work day, even though pediatricians say adolescents are hard-wired to fall asleep and wake up later and need more sleep.

It doesn’t just mean that teenagers are nodding off in class. The Centers for Disease Control and Prevention have called insufficient sleep among the nation’s adolescents “a substantial public health concern” and said schools should do their part to encourage teens to get more rest. Teens who don’t get enough sleep are more likely to do poorly in school, be overweight and try tobacco, alcohol and other drugs. Teens operate on different circadian rhythms, making it difficult for them to fall asleep before 11 p.m., doctors say.

Some large school districts have taken heed and moved start times closer to the recommendation, but many have found it too logistically challenging and expensive to consider.

Virginia’s Fairfax County, one of the nation’s largest school systems, pushed start times from 7:20 a.m. past 8 a.m. for high school students in 2014, but in turn had to open middle schools earlier. It cost the district $4.9 million.

Montgomery County in Maryland also changed bell times for high schoolers in 2015, a move that contin ues to be controversial. High school start times moved from 7:25 a.m. to 7:45 a.m. under the plan.