Numerous studies have looked at how the amount of sleep children get changes as they age, but there has been little research into when they get that sleep and how that evolves over time.

A team of researchers from Brown University and the Lifespan health system set out to change that by arming 94 children ages 9 to 18 with activity monitors to track how their sleep habits changed during a two-year-period. They found that children were going to bed later and later but had to wake up at a fixed time every weekday to make it to school on time. Their conclusion: "[C]hildren seem fated to get less sleep as they mature."

The issue of how much sleep children need and are able to get in today's modern world has been a hot topic in recent years with many parents calling for later school start times for middle and high schools. Children are believed to need about nine hours of sleep during adolescence but many are not getting that minimum, and many parents and health-care professionals blame school bell times that are too early.

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The American Academy of Pediatrics has recommended that schools start after 8:30 a.m. based on the growing body of research about the negative effects of insufficient sleep for teens. Studies have shown that it not only affects academic performance but can lead to mood disturbances and obesity. In Fairfax County, for instance, the school board has recommended that high schools start later, between 8-8:10 a.m., beginning in the 2015-16 school year. The members said the reason for the change is that high school is "a crucial period for students’ college-preparatory or work-readiness years as well as their athletic engagements and other activities." In Montgomery County, parents have protested a decision by the superintendent earlier this year to scrap a proposal to move start times later due to cost.

The sleep patterns study, published Friday in the journal PLoS ONE, found that the typical 9-year-old would fall asleep at 9:30 p.m. on weekdays but by age 11 that same child would sleep at 10 p.m. The child continued to wake up at 6:40 a.m. in order to make it on time for school -- resulting in missing a half hour of sleep every night. The researchers said they found a similar pattern for teens who were 15 to 17. On the weekends, most of the children got considerably more sleep by sleeping in in the mornings.

"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 researchers wrote.

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That pattern changed dramatically when many of the study participants turned 18. The study found that sleep times spiked for that age group during the weekdays. The explanation, the researchers noted, is likely very simple: "They had graduated from high school."

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