We all need sleep. It’s a core part of being human, consuming up to a third or more of each day. Without sleep, basic brain processes like attention and memory, the ability to learn, and our overall well-being go haywire. But over the past century, the average amount of sleep for American school-aged children and adolescents has dropped by about 1 hour to just under 7 hours.

This lost hour hits teenagers the hardest, and the detrimental effect of sleep deprivation on the physical and mental health of teenagers has been extensively studied. This is because adolescence is a time filled with all sorts of bodily changes, increased independence, and the emergence of new social roles—all affecting health and behavior. Bedtimes naturally become later with each passing year as a teen grows older. On the other hand, rise times, which are more often determined by school start times, remain unchanged.

Along these lines, a simple intervention has been proposed to increase teen sleep—delay school start times. In the US, this proposal has been backed by the CDC, which issued a statement in 2018 proposing that start times be moved to 8:30 or later. This concept has been tested in various settings. Several school districts in the US and around the world have indicated that changing school start times changed teen sleep. For example, a study in one Rhode Island School District that transitioned from a 08:25 a.m. start time in ninth grade to a 7:20 a.m. start time in 10th grade confirmed a shortening of daily sleep of approximately 20 min. In Singapore, a recent study showed that a delay of 45 min in start times in one all-girls high school resulted in a modest (about 10 minutes) increase in daily sleep.

Although school districts, cities, and states around the world have brought the matter to the table, why hasn’t this basic strategy to increase teen sleep by delaying school start times been more widely implemented? One reason is a host of practical considerations. For example, parents have concerns about a later start affecting sports and other after-school activities, and also about after-care for younger children. (It’s common for districts to suggest swapping the start times of older and younger kids, so busses can serve all them.) Another reason for not implementing delayed school start times is that this connection between school start times and sleep has fallen into the shadows of the debate on widespread regular use of digital technologies by young people and its negative impact on psychological well-being. However, a recent study showing that screen use is as predictive of the amount teenagers sleep as how many potatoes they eat—bullying and marijuana use were the most linked to teen sleep times—should re-shift the conversation back to the impact of school start times on sleep.

A recent study in the UK on 120,000 15-year-olds pointed to the dangers of inadequate sleep, with the authors claiming that sleep is the strongest predictor of wellbeing in teenagers. Yet, the links between delaying school start times and teenage biological clocks, school performance, and physical and mental health have been hard to establish. But in a massive collaborative research effort, scientists worked with the Seattle School District to test how delaying school times affected teen sleep duration, academic performance, and alignment with the body’s natural clock. They found that an increase in the daily median sleep duration is associated with an increase in average grades and an improvement in attendance.

Neuroscientists and biologists from the University of Washington and the Salk Institute conducted a pre-/post-study—where participants are studied before and after the experimental manipulation—in which they measured activity, light, and sleep-wake cycles. To do so, the school district delayed the start time for two high schools over the course of two years: from 07:50 a.m. in the academic year 2015-2016 to 08:45 a.m. for the 2016-2017 academic year. The delay in schools’ start time of 55 minutes led to an increase of about 35 minutes in average daily sleep duration from 6 hours and 50 min to 7 hours and 24 min—restoring historical sleep values in teenagers before evenings with brightly lit environments and access to light-emitting screens were common.

The increase in daily sleep appears to have had cascading positive effects. Later school start time was associated with a better alignment of sleep timing with the circadian system and reduced sleepiness, as well as improved academic performance with better grades. Both high schools saw better attendance, but one of the two high schools reported significantly fewer absences and increased punctuality in 2017 than in 2016; notably, this high school has many more economically disadvantaged students (88%) and ethnic minorities (68%) enrolled than the other high school (31% and 7%, respectively). In summary, delaying high school start times brings students closer to reaching the recommended sleep amount and reverses the century-long trend in gradual sleep loss, and affects high school grades, attendance, and punctuality.

What remains largely unknown is whether interventions designed to improve sleep patterns among teenagers may also prevent or delay the onset of mental disorders. Many questions regarding the connection between teenage sleep deprivation and lifelong mental health remain unanswered. Longitudinal studies—research involving repeated observations of the same people over periods of time—are needed investigating the effects of adolescent sleep patterns and the risk for developing mental disorders across the course of life.

Still, the ‘Sleepmore in Seattle’ study is already having an impact. School boards in Orlando and Philadelphia are already looking at this example to see if it should make similar changes. In France, backed by advocates pushing the study’s findings, Parisian teenagers will be allowed an extra hour’s sleep before school in a move to improve their health. Ironically, whether the Seattle School District will push for a permanent change in school start times remains to be seen.