Ramsgate Arts Primary School, in the county of Kent, made a decision that many parents and principals considered radical: moving the start time from 8:50 to 10:00 in the morning. The justification was not a whim but biology. Studies on adolescent sleep indicate that the circadian clock of young people between 12 and 18 years old is programmed to wake up later, which means that forcing them to perform at 7 or 8 in the morning is equivalent to asking an adult to work at 4 a.m. The experiment lasted a full school year, and the numbers were striking: illness-related absences dropped by 55% and overall grades improved steadily. What seemed like a convenience turned out to be a public health intervention. The uncomfortable fact is that most education systems in the world continue to ignore this evidence.