California high schoolers returning to class can look forward to a welcome back-to-school gift: more time to sleep. A new state law will require that public high schools start classes no earlier than 8:30 a.m., a half-hour later than the national average. The shift has the potential to improve students’ mental health and academic performance — so long as schools commit the time and money necessary to make it work.

For much of the 20th century, schools opened their doors at 9 a.m., in line with most businesses. Yet today, more than 80% of high schools in the U.S. start before 8:30 a.m., and nearly half before 8 a.m. — a norm dictated by convenience more than pedagogy. Pushing up start times for older students allows school districts to save on transportation costs by running staggered bus schedules. It also means an earlier end to the school day, leaving sufficient daylight for non-academic activities, particularly organized sports.

The problem is that starting school too early may do teenagers more harm than good. As a rule, adolescents don’t get enough sleep — and not just because they’re intentionally avoiding it. Among many other physiological changes brought on by puberty, the body’s circadian rhythms shift, causing teenagers to stay awake later. Smartphones, social media and video games are likely exacerbating that tendency, in part because excessive screen time interferes with the brain’s ability to shut down at night.

Experts say teens’ alertness levels in morning classes are comparable to those of adults roused from sleep at 4 a.m. The American Academy of Pediatrics and the American Academy of Sleep Medicine advocate pushing back school start times for adolescents. Places that have done so have seen gains in both students’ sleep and their academic achievement; a Seattle study showed that students who had later start times earned higher grades than those who kept the earlier schedule. A RAND analysis found that, thanks to improved student performance and reduced car crashes, delaying school start times to 8:30 a.m. would add $83 billion to the U.S. economy over 10 years.

Given the weight of such data, it’s no wonder states are rethinking things. New Jersey could well follow California’s lead. The legislature in New York, home of the nation’s largest school district, is considering a bill that would make 8:30 the start time across all K-12 grades. Several other states encourage school districts to adopt later start times, but don’t require it.

Although such experimentation is welcome, states should be wary of potential complications and drawbacks.

One is that getting to school on time has always been a basic requirement for all students, and should remain so. Rising at a regular hour each day teaches young people discipline, personal responsibility and respect for others — skills that will yield benefits throughout their lives, regardless of the educational or professional paths they pursue. Later start times should not mean punctuality is optional.

School leaders will also need to make smart choices about how to allocate resources. Changes in bus schedules may require some districts to increase the size of their fleets or hire additional drivers. After-school programs may need to be expanded for younger students. To ensure student athletes get home at a reasonable hour, schools should experiment with running outdoor sports and other extracurriculars before school. Rural districts that require long commutes should be given flexibility to adjust school schedules during winter months, when some students might have to wait for transportation in the dark. California built in a three-year window for schools to transition to an earlier start, giving districts a chance to share best practices along the way. Lawmakers should be attentive to these efforts, and ready to pull the plug if things go wrong.

Some perspective is also in order. Helping U.S. students recover the learning lost to the pandemic will require a national commitment to improving the quality of public schools and the instruction they provide. A little extra sleep, on its own, won’t be enough to boost every high schooler’s educational and social fortunes. But it’s a start.

More From Bloomberg Opinion:

• Half-Time High School May Be Just What Students Need: Michael Petrilli

• A Wake-Up Call for Public Education: Michael Bloomberg

The Editors are members of the Bloomberg Opinion editorial board.

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