A recent study by the Centers for Disease Control and Prevention has found that most U.S. middle and high schools start the school day too early, before 8:30 a.m.

Lead author Dr. Anne Wheaton says if teens don't get enough sleep, they are more prone to not getting enough exercise and engaging in risky behavior like drinking alcohol. She cites a recommendation from the National Sleep Foundation that 14 to 17-year-olds need eight to 10 hours of sleep a night.

One of the first things to change in puberty, Dr. Wheaton says, is a delay in the "sleep cycle,"

or the time at which kids start to feel sleepy. She says "kids really can’t fall asleep before, maybe, 11 o’clock at night."

Wheaton joins Here & Now's Robin Young to discuss the study findings and explain why students need more sleep.

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