Arne Duncan, US Secretary of State for Education, tweeted in 2013: 'let teens sleep, start school later'. This paper examines early starts and their negative consequences in the light of key research in the last 30 years in sleep medicine and circadian neuroscience. An overview of the circadian timing system in adolescence leading to changes in sleep patterns is given and underpins the conclusion that altering education times can both improve learning and reduce health risks. Further research is considered from education, sleep medicine and neuroscience studies illustrating these improvements. The implementation of later starts is briefly considered in light of other education interventions to improve learning. Finally, the impact of introducing research-based later starts synchronized to adolescent biology is considered in practical and policy terms.Keywords: education; adolescence; school start times; circadian; sleep deprivation; learning; health Introduction Time is a fundamental variable in human biology and in education, yet currently the two disciplines measure time in different ways. For the educator, student development is defined by age and the daily timetable by social conventions that vary between countries, regions and even individual schools. In contrast, biological time is measured in developmental changes in the body, and over the day by our internal biological clock. It is no surprise given the relative novelty of mechanical clocks in evolutionary timescales that our ability to function optimally, including in learning, varies with biological time rather than conventional social times.