When Seattle Public Schools announced that it would reorganize school start times across the district for the fall of 2016, the massive undertaking took more than a year to deploy. Elementary schools started earlier, while most middle and all of the district's 18 high schools shifted their opening bell almost an hour later—from 7:50 a.m. to 8:45 a.m.

Parents had mixed reactions. Extracurricular activity schedules changed. School buses were redeployed. And as hoped, teenagers used the extra time to sleep in.

In a paper published Dec. 12 in the journal Science Advances, researchers at the University of Washington and the Salk Institute for Biological Studies announced that teens at two Seattle high schools got more sleep on school nights after start times were pushed later—a median increase of 34 minutes of sleep each night. 

This boosted the total amount of sleep on school nights for students from a median of six hours and 50 minutes, under the earlier start time, to seven hours and 24 minutes under the later start time.

The sleep duration of students

"This study shows a significant improvement in the sleep duration of students—all by delaying school start times so that they're more in line with the natural wake-up times of adolescents," said senior and corresponding author Horacio de la Iglesia, a UW professor of biology.

The study collected light and activity data from subjects using wrist activity monitors—rather than relying solely on self-reported sleep patterns from subjects, as is often done in sleep studies—to show that a later school start time benefits adolescents by letting them sleep longer each night.

The study also revealed that, after the change in school start time, students did not stay up significantly later: They quietly slept in more extended, a behavior that scientists say is consistent with the natural biological rhythms of adolescents.

"Research to date has shown that the circadian rhythms of adolescents are simply fundamentally different from those of adults and children," said lead author Gideon Dunster, a UW doctoral student in biology.

In humans, the churnings of the circadian rhythms help minds and bodies maintain an internal "clock" that tells us when it is time to eat, sleep, rest and work on a world that spins once on its axis approximately every 24 hours.

The genes and external cues from the environment, such as sunlight, combine to create and maintain this steady hum of activity. But the onset of puberty lengthens the circadian cycle in adolescents and also decreases the rhythm's sensitivity to light in the morning.