When high schools start at 8.30am or later, attendance rates and graduation rates improve, according to a new study.

The study backs previous research that says additional sleep boosts psychological, behavioral and academic benefits for teens.

"So much research explains the impact of insufficient sleep on suicide, substance abuse, depression, auto accidents and more," said lead study author Pamela McKeever of Central Connecticut State University in New Britain.

"This connects the dots between the world of science and education," she told. "Through this, educators and parents can see how lack of sleep impacts the school indicators that we use to measure student success."

McKeever and colleague Linda Clark looked at school start times, graduation rates and attendance rates for 30,000 students in 29 high schools across seven states. They found that two years after a delayed start was implemented at these high schools, average attendance rates and graduation rates had increased several percentage points.

For example, the average graduation completion rate was 79 per cent before the delayed start was implemented, and it was 88 per cent afterward.

This doesn't only impact our high school students. This impacts all of society

"This doesn't only impact our high school students. This impacts all of society," McKeever said. "As graduation rates improve, young adults experience less hardship after graduation, a lower chance of incarceration and a higher chance of career success."

Delayed bell times could close the achievement gap as well, McKeever and Clark wrote in Sleep Health, the journal of the National Sleep Foundation in the UK. When schools start later, students in lower socioeconomic categories are more likely to get to the bus on time. When they arrive at school on time, they're more likely to stay in class and graduate.

"When kids miss a bus early in the morning and that's their only form of transportation, they miss class and then soon the credits," said Kyla Wahlstrom of the University of Minnesota in Minneapolis, who wasn't involved with this study. "People don't understand the link between early wake-up times and graduation rates, but it's that direct."

Since the late 1990s, Wahlstrom and other researchers have suggested that delayed high school start times may help students. In 2014, she and her colleagues reported that in a three-year study with 9,000 students in eight public high schools across three states, attendance rates increased with a start time of 8.35am or later.

In December, the American Academy of Sleep Medicine advised that later school start times could improve sleep, reduce car accidents and reduce sleepiness. The American Academy of Pediatrics also recommends 8.30am as the earliest time to begin school.

But school policies have yet to change nationwide. The US Centers for Disease Control and Prevention (CDC) reports that in 42 states, 75-100 per cent of public schools start before 8.30am.

Teens are "driven by biology to go to sleep later, and there's not much we can do about that, but school start times are the main reason they get up when they do," said Anne Wheaton, an epidemiologist at the CDC in Atlanta. Wheaton wasn't involved with this study.

A limitation of the study is that many variables affect attendance and graduation rates. Changes at the school level, such as different teachers, policies and the surrounding community itself, could affect students and their ability to complete class credits, extracurricular activities and afterschool jobs. Also, the data didn't measure sleep time or indicate whether students slept more due to delayed start times.

"The debate about school start time and adolescent sleep patterns has been going on for a number of years," said Mary Carskadon of the Sleep for Science Research Lab at Brown University in Providence, Rhode Island, who wasn't involved with this study.

This includes having a set bedtime and limiting arousing activities in the evening

"Efforts to delay the school bell are more likely to succeed best when parents and the teens themselves use better choices," she told. "This includes having a set bedtime and limiting arousing activities in the evening."