Circadian Rhythms Shift for Later Sleep, but School Remains Early for Teens

By following dozens of younger and older adolescents for more than two years, researchers in a new study were able to determine that the children fell asleep later and their circadian rhythms shifted later as they grew older. But early school start times interfere with their tendency to sleep later, reducing their total sleep. The study bolsters new recommendations for later school start times.

A newly published study used activity monitors to track how sleep habits changed in younger and older teens as they grew during a two-year period. Key findings, for instance that the children fell asleep later as they matured and resisted sleep longer after the nightly onset of hormonal sleep signals, lend new support to recent recommendations by the American Academy of Pediatrics that middle and high schools avoid starting earlier than 8:30 a.m.

Those recommendations, published in August, were based on a deep body of research evidence that although children require about nine hours of sleep throughout adolescence, older kids are naturally inclined to stay up later than younger ones. The new study, based at Brown University and the Lifespan health system, uniquely adds to that evidence. It appears in the journal PLoS ONE.

“This is one of the few studies that has tracked sleep behavior and circadian rhythms over the course of up to two-and-a-half years in the same adolescents,” said lead author Stephanie Crowley, an assistant professor at Rush University Medical Center in Chicago who was a graduate student in the Brown University lab of senior author Mary Carskadon when the study was conducted. “A handful of studies have focused on sleep duration longitudinally [as children age] but this is one of the first to look at sleep timing longitudinally.”

This image shows a teenage girl sleeping on her school bag.
A body at rest (wants to) remain at rest. If school starts early in the morning and adolescent physiology naturally produces later bedtimes, the inevitable result is less sleep. A new study adds to the case for later school start times for middle and high school students. Credit Mike Cohea.

Tracking the changing sleep patterns of children as they age is important because numerous studies have found that teenagers don’t get enough sleep. The negative effects researchers have found can include poor academic performance, mood disturbances, depression, obesity, and even drowsy driving accidents among older teens. Those are the kinds of problems that concerned the American Academy of Pediatrics.

Later to bed, still early to rise

According the data in the new study, conducted with 94 children in all, children seem fated to get less sleep as they mature. A typical 9-year-old fell asleep at 9:30 p.m. on a weekday upon first enrolling in the study and would wake up at 6:40 a.m. By age 11, the same child would go to sleep at 10 p.m. but still awaken at the same time. The net result for that child — and many others in the cohort of 38 children who joined the study at 9 or 10 years old — would be the loss of about half an hour of sleep every night.

“There are changes in sleep,” Crowley said, “even as early as middle school.”

In another example typified by the data, a volunteer who enrolled at age 15 going to sleep at 10:35 p.m. on a weekday and waking at 6:20 a.m., would leave the study at age 17 going to sleep at 11:05 p.m. and waking at 6:35 a.m. In other words, weekday sleep time continued to shrink with age for the 56 volunteers aged 15 or 16 who participated in the research.

What of weekends? The volunteers got considerably more sleep than on weekdays, despite falling asleep later and later with age. That’s because were able to sleep later.

The typical kid in the study moved to a later sleep/wake cycle with aging, except during the week, when later waking was not possible, most likely because of early school bells. The 18-year-olds in the study slept dramatically longer on weekdays than they did when they were younger because they had graduated from high school.

The study is one of the few to track individual kids for longer than a year. It illustrates wide individual differences in these trajectories. For some children, the data show, the shift to a later bedtime without a later wakeup time was especially stark, possibly putting them at a greater disadvantage relative to their peers.

“We often say ‘on average this is what’s happening,’ but there are youngsters who are more devastated by the kinds of issues they confront with school start times that are imposed on them,” said Carskadon, professor of psychiatry and human behavior in the Warren Alpert School of Medicine at Brown and director of the Sleep Research Center at the Bradley Hospital in East Providence. “These individual differences do change the impact of interventions [such as starting school later].”

‘Resisting’ a rest

Carskadon and Crowley’s team measured another important difference in their younger and older cohorts of volunteers: a difference in how long the children resisted the tell-tale signal of biological night — the rise of the hormone melatonin as measured in their saliva. This interval between the time when the biological sleep signal emerged and when the children fell asleep is important because the longer kids resist that signal, the groggier they will be when they wake up in the morning.

The data showed that as teens got older they put off the signal up to an hour longer, staying up as long as two hours past the point when their brains became primed for sleep. By the following weekday morning, however, there had been less time to relieve that accumulated “sleep pressure.”

“They were going to sleep at a later time in the biological night,” Crowley said.

Carskadon continued: “They’ll have an easier time falling asleep because they are so much past when the gate for sleep opens — but when they still have to get up in the morning, that’s a problem.”

About this neurodevelopment research

In addition to Carskadon and Crowley, the paper’s other authors are Eliza Van Reen, Christine Acebo, Leila Tarokh, Ronald Seifer, and David Barker of Brown and the Bradley Hospital; and Monique LeBourgeois of the University of Colorado.

The National Institute on Alcohol Abuse and Alcoholism supported the research.

Contact: David Orenstein – Brown
Source: Brown press release
Image Source: The image is credited to Mike Cohea and is in adapted from the Brown press release
Original Research: Full open access research for “A Longitudinal Assessment of Sleep Timing, Circadian Phase, and Phase Angle of Entrainment across Human Adolescence” by Stephanie J. Crowley, Eliza Van Reen, Monique K. LeBourgeois, Christine Acebo, Leila Tarokh, Ronald Seifer, David H. Barker, and Mary A. Carskadon in PLOS ONE. Published online November 7 2014 doi:10.1371/journal.pone.0112199

Open Access Neuroscience Abstract

A Longitudinal Assessment of Sleep Timing, Circadian Phase, and Phase Angle of Entrainment across Human Adolescence

The aim of this descriptive analysis was to examine sleep timing, circadian phase, and phase angle of entrainment across adolescence in a longitudinal study design. Ninety-four adolescents participated; 38 (21 boys) were 9–10 years (“younger cohort”) and 56 (30 boys) were 15–16 years (“older cohort”) at the baseline assessment. Participants completed a baseline and then follow-up assessments approximately every six months for 2.5 years. At each assessment, participants wore a wrist actigraph for at least one week at home to measure self-selected sleep timing before salivary dim light melatonin onset (DLMO) phase – a marker of the circadian timing system – was measured in the laboratory. Weekday and weekend sleep onset and offset and weekend-weekday differences were derived from actigraphy. Phase angles were the time durations from DLMO to weekday sleep onset and offset times. Each cohort showed later sleep onset (weekend and weekday), later weekend sleep offset, and later DLMO with age. Weekday sleep offset shifted earlier with age in the younger cohort and later in the older cohort after age 17. Weekend-weekday sleep offset differences increased with age in the younger cohort and decreased in the older cohort after age 17. DLMO to sleep offset phase angle narrowed with age in the younger cohort and became broader in the older cohort. The older cohort had a wider sleep onset phase angle compared to the younger cohort; however, an age-related phase angle increase was seen in the younger cohort only. Individual differences were seen in these developmental trajectories. This descriptive study indicated that circadian phase and self-selected sleep delayed across adolescence, though school-day sleep offset advanced until no longer in high school, whereupon offset was later. Phase angle changes are described as an interaction of developmental changes in sleep regulation interacting with psychosocial factors (e.g., bedtime autonomy).

“A Longitudinal Assessment of Sleep Timing, Circadian Phase, and Phase Angle of Entrainment across Human Adolescence” by Stephanie J. Crowley, Eliza Van Reen, Monique K. LeBourgeois, Christine Acebo, Leila Tarokh, Ronald Seifer, David H. Barker, and Mary A. Carskadon in PLOS ONE. doi:10.1371/journal.pone.0112199.

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