Later Bedtimes Start in Adolescence

People start sleeping later in their teenage years, according to a new study.

After tracking the sleep habits of teens during a two-year period, researchers found that children fell asleep later as they matured and resisted sleep longer after the nightly onset of hormonal sleep signals.

Because children require about nine hours of sleep throughout adolescence and older kids tend to stay up later than their counterparts, researchers recommend that middle and high school avoid starting earlier than 8:30am.

"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," 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, said in a news release. "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."

Previous studies revealed that most teens don't get enough sleep. This is worrying because sleep deprivation can negatively affect grades, mood, weight and even driving performance.

"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," 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, said in a news release. "These individual differences do change the impact of interventions [such as starting school later]."

The findings are published in the journal PLoS ONE.