Beginning this fall, the Philadelphia School District is shifting its high school start times to a mostly universal 9 a.m., reflecting a national movement to improve academic outcomes by allowing teens more sleep.

A letter sent to Philadelphia School District principals and staff Thursday by chief of schools Evelyn Nuñez says, “Research shows that later start times for high school students enable them to arrive at school more alert and ready to learn.”

The decision comes after the school system created earlier start times this year, some to 7:30 a.m. Before the pandemic, high school start times varied.

The changes this year were made to standardize bell schedules in response to transportation concerns and a nationwide bus driver shortage, and were generally met with widespread frustration. Officials said at the time they would take community input into consideration in making future changes.

Teens’ sleep patterns shift later when they go through puberty, and they need more sleep to function well, said Ariel Williamson, a psychologist and researcher at Children’s Hospital of Philadelphia who studies sleep. If a student had to be up at 6 a.m. for school, they would have to go to bed by 10 p.m. to get eight hours of sleep, the minimum their bodies need to function well.

“That’s just so much earlier than their circadian rhythms allow,” said Williamson.

Arun Handa, a CHOP psychiatrist and sleep medicine physician, said the change would likely improve student performance.

“If you’re tired and you’re going to school, you might be falling asleep during first period,” Handa said. “If it’s delayed a little bit, you may be paying attention, be making it to school on time.”

Monica Lewis, a spokesperson for the district, said in a statement that the district got community feedback in the spring and relied on American Academy of Pediatrics research to make the call to move start times later for high schoolers.

“In addition, since a majority of our high schoolers are responsible for making their own way to school, later start times promote safer travel in the mornings and afternoons,” Lewis said.

But though the changes are likely to result in better learning outcomes, they could have unintended, negative consequences for Philadelphia, some high school teachers and principals said — hurting students who work and families who rely on that income, and reducing participation in extracurricular activities with parents reluctant to allow their children to make their way home through dangerous neighborhoods later at night.

“It’s devastating,” said one teacher, who asked not to be identified for fear of reprisal. “I’m beside myself at the ramifications of this.”

The teacher, who said he understood the developmental and academic reasons for the switch, said he already hears from parents anxious about their kids making their way home safely after practices or games. Moving start times later will “decimate” extracurriculars, he said.

When the final bell of the day rings, many students “are sprinting out of the building because they have to get to that job or they get in trouble, and their families need that income,” the teacher said. “We’re going to be losing so many kids who are going to be forced to pick between school and supporting their families.”

And those who might forego sports or clubs because of safety or childcare concerns will lose out not just on the meaningful experience of participating in extracurricular activities but also potential higher-education opportunities.

“Most of our students rely on those opportunities to get to college because they thrive on the athletic field or in clubs like robotics,” said the teacher, who also serves as a coach.

The 9 a.m. start means schools will end at 4:04 p.m.

The changes will affect all schools with high school grades, including those that also contain middle schools, like Masterman, Girard Academic Music Program and George Washington Carver High School of Engineering and Sciences. Exempt from the changes are four schools that work with partner institutions — Widener Memorial School, Pennypack House School, Philadelphia Juvenile Justice Services Center, and Parkway Center City Middle College High School.

Pre-K programs located in high schools will keep their current schedules.

Though it has already made the decision to shift start times, the district will hold virtual community forums around the upcoming changes beginning next week, Lewis said, aimed at providing feedback “around the supports students and families may need as they plan ahead for the 2022-2023 school year with the bell schedule adjustments in mind.”

Williamson, the CHOP sleep researcher, said the change was a good one, particularly in light of how tough the pandemic has been for adolescent mental health.

“If there’s anything that we can do to improve the well-being of teens,” Williamson said, “this is one thing that could start to get us there.”