A new study adds to the growing evidence that teens get more sleep when schools starts later.

The findings, published in the journal Science Advances, show that students at two Seattle high schools got significantly more sleep after start times were pushed to almost an hour later.

This boosted the total amount of sleep on school nights 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.  That represents a median increase of 34 minutes of sleep each school night.

Importantly, after the change in school start time, students did not stay up significantly later; they simply slept in longer, a behavior that scientists say is consistent with the natural biological rhythms of adolescents.

“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 Dr. Horacio de la Iglesia, a professor of biology at the University of Washington (UW).

For the study, researchers from UW and the Salk Institute for Biological Studies collected data from subjects using wrist activity monitors rather than relying solely on self-reported sleep patterns, as is often done in sleep studies.

“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.

The human circadian rhythm allows our minds and bodies to maintain an internal “clock” that tells us when it is time to eat, sleep, rest and work in a world that spins once on its axis approximately every 24 hours.

But the onset of puberty lengthens the circadian cycle in adolescents and also decreases the rhythm’s sensitivity to light in the morning. These changes cause teens to fall asleep later each night and wake up later each morning relative to most children and adults.

“To ask a teen to be up and alert at 7:30 a.m. is like asking an adult to be active and alert at 5:30 a.m.,” said de la Iglesia.

Sleep experts generally recommend that teens get eight to 10 hours of sleep each night. But early-morning social obligations — such as school start times — force adolescents to either shift their entire sleep schedule earlier on school nights or truncate it.

“All of the studies of adolescent sleep patterns in the United States are showing that the time at which teens generally fall asleep is biologically determined — but the time at which they wake up is socially determined,” said Dunster.

“This has severe consequences for health and well-being, because disrupted circadian rhythms can adversely affect digestion, heart rate, body temperature, immune system function, attention span and mental health.”

For the study, researchers compared the sleep behaviors of two groups of sophomores, all enrolled in biology classes at Roosevelt and Franklin high schools.

One group of 92 students, drawn from both schools, wore wrist activity monitors all day for two-week periods in the spring of 2016, when school still started at 7:50 a.m. The wrist monitors collected information about light and activity levels every 15 seconds, but no physiological data about the students.

In 2017, about seven months after school start times had shifted later, the researchers had a second group of 88 students, again drawn from both schools, wear the wrist activity monitors.

Two teachers at Roosevelt and one at Franklin worked with the UW researchers to carry out the study, which was incorporated into the curriculum of the biology classes. Students in both groups also reported their sleep data.

The data obtained from the wrist monitors revealed a significant increase in sleep duration, due largely to the effect of sleeping in more on weekdays.

“Thirty-four minutes of extra sleep each night is a huge impact to see from a single intervention,” said de la Iglesia.

The findings show other improvements as well. After the change, the wake-up times for students on weekdays and weekends moved closer together.

Also, their academic performance improved in biology class: Final grades were 4.5 percent higher for students who took the class after school start times were pushed back compared with students who took the class when school started earlier.

In addition, the number of tardies and first-period absences at Franklin dropped to levels similar to those of Roosevelt students, which showed no difference between pre- and post-change.

Source: University of Washington