School began in most districts across the country today. Many high schools start at 7:30 AM....but in Minneapolis, high schools have, for the fourth year, begun their day later...at 8:40AM. The dramatic change is the result of years of sleep research on adolescents...which finds they naturally fall asleep later, around 11:30PM. And what's more...they need about nine and a half hours of sleep each night. So without the later start times..teens are perpetually in what researchers call "sleep debt." NPR's Michelle Trudeau reports.
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