Ideally, the American Academy of Pediatrics says, the first bell should ring at 8:30 a.m. or later -- which is the case at only 15 percent of U.S. high schools right now.
At the very least, classes should start no earlier than 8 a.m., said Dr. Judith Owens, the lead author of a new academy policy statement on school start times.
The recommendations, published in the academy's journal Pediatrics, are based on research showing that U.S. kids are sleep-deprived, which has consequences for their health, school performance and safety.
A new study calls on schools to start later to increase teens' potential.
"This is an important issue," said Dr. Marcel Deray, a Florida sleep specialist who wasn't involved in the recommendations.
"I see a lot of teenagers who are tired and have problems in school because they have to get up so early," said Deray, who directs the Sleep Disorders Center at Miami Children's Hospital. "Some kids are getting up at 5 a.m., 6 a.m."
Many people think the answer is for kids to just get to bed earlier, Owens noted. But it's not that easy, she said, because biology has other plans.
Around puberty, the body's natural sleep-wake cycle shifts, and it's actually hard for teenagers to fall asleep earlier than 11 p.m.
"Teenagers' bodies release melatonin later than (adults') do," Deray explained, referring to a hormone the brain secretes in the evening to induce drowsiness.
"The other issue," Owens said, "is that teenagers' sleep needs are greater than many people think. They need nine to nine-and-a-half hours."
Yet, 43 percent of U.S. public high schools start classes before 8 a.m., according to the U.S. Department of Education. Middle schools, meanwhile, typically start classes at 8 a.m. -- with about 20 percent starting earlier than that.
"And that's the first bell," Owens said. "That's not even counting the commute time."
So in areas where kids take the school bus, the actual school day could begin before sunrise.
According to Owens, U.S. high schools started bumping up start times back in the 1950s and '60s, mainly to space out bus runs. Many school districts start high schools and middle schools first, then circle buses back to pick up elementary school kids.
One option would be to flip elementary and high school start times, but parents often oppose that -- since young children could be standing at bus stops in the early-morning dark, or have no one at home after school. Another option -- running more buses -- would be expensive.
Transportation is not the only obstacle, according to the National Sleep Foundation, which supports pushing back school start times. Some other arguments are that sports and extracurricular activities would end too late; teenagers would have no time for after-school jobs; and a later start could conflict with working parents' schedules.
Owens acknowledged the logistical challenges, and said parents and school staff sometimes oppose later start times. As an example, she said the Fairfax County, Va., school district has been debating the issue for years. Four options, including one with a 9:15 a.m. high school start, will be put to a vote this fall.
Some districts have found "creative solutions" to certain obstacles -- like having high school students switch from school buses to public ones, according to the academy.
Deray thinks tackling the practical challenges is worthwhile. He said studies have linked kids' sleep deprivation not only to poorer school performance, but also to higher rates of car accidents, obesity and depression.
In a recent study of eight U.S. high schools that delayed their start times, researchers found improvements in kids' grades, attendance and car crash rates.
Later school starts are just one way to help sleepy kids, however. Deray offered parents some tips.
"All electronic devices should be turned off an hour or two before bed," he said. "The blue light from them suppresses melatonin production."
He also advised avoiding physical and mental stimulation close to bedtime -- which means exercise and homework should be done earlier in the evening. And while teenagers love to sleep till noon on weekends, that will only further disturb their sleep/wake cycle. Deray said kids should get up by 9 a.m. on weekends and get plenty of morning sunlight.
He acknowledged, though, that all of that is easier said than done.
Owens agreed. "One of the issues is that kids are overscheduled," she said. "Don't give them so many things to do that they can't get to bed before midnight."